Is the basic premise of transcendental idealism still valid?

Logically speaking, the argument for it would be just as valid or invalid now as it was in Kant's day. If you mean 'Is it still relevant?' then I'd say only for the religious-minded. It's not a good companion for scientific reductionism.

If what you mean by "contemporary philosophy" is reflected in the work of the neo-pragmaticists and post-modernists then Kant was a watershed (via phenomenology) for the devaluation of the term "reality" as the epitome of "objectivity". However it would take an essay to fully develop that theme. (see for example Rorty: Philosophy and the Mirror of Nature)

I therefore agree with FBM that the concept "reality" as "an ultimate state" (noumema), now lies in the province of pseudo-religious speculation, and that scientists are de facto engaged in the exploration of workable yet potentially transient paradigms (Kuhn) which in essence bypass the lay term "reality".

As Werner Heisenberg said ....

italics mine.

(Are you James Collins as in philpapers ?)

i think that the notion of "validity", in general, is being reevaluated today, as happens occasionally. However, i do think that the conceit of 'transcendental idealism" is still of current interest, if filtered through various contemporary -isms.

And regardless of contemporary attitudes regarding them, Kant's writings remain a watershed moment in Φ history, and those that neglect them do so at their peril.

I don't see what religion has to do with it. His philosophy is certainly compatible with religious belief but the logic of his arguments doesn't rest on an appeal to assumptions of a religious nature.

No, I'm not.

The noumenon required by Kant is an entity/form not perceived by the senses, much like a god or spirit or soul. Kant's philosophy/logical argumentation doesn't actually need religion (he wouldn't agree with that statement, probably), but Christian apologists have long looked to him and Plato as ad hoc justifications for faith. Somebody more up on Kant might be able to do a better job of explaining it than I.

I agree.

I can't say I've read much contemporary philosophy and maybe that's because it doesn't really offer a systematic view of reality as a whole or attempt to answer the "big" questions (or at least that's how it seems to me). As a non-academic my interest in philosophy has a practical motivation. I'm seeking answers to questions of existential importance such as the ultimate nature of reality.
Maybe one possible explanation for the current state of affairs in philosophy with regard to the point I've just made, is that it is a direct result of Kant's ideas (in which case I just answered my original question). That is, the conclusion arrived at by Kant was that reality as it exists in itself is ultimately unknowable and so nothing meaningful can be said about it. I think Wittgenstein was basically arguing the same point. Of course, as someone with a limited background in philosophy I realize this is a gross oversimplification but that's why I'm here.

@FBM Point taken, but I just don't get why a philosophy should be automatically discounted or somehow refuted just because it happens to lend support to someone's religious beliefs. As far as I'm concerned that's beside the point. Kant's noumenon is not argued for in a positive sense. It's a position he arrived at on the basis of logical reasoning. Isn't philosophy essentially speculative anyway?

When I said it was probably only still mostly relevant to the religious-minded, that was just an observation, not a condemnation. I'm pointing out that much of speculative philosophy's ponderings have become obsolete in the course of the development of advanced scientific investions. Speculating and arguing about whether or not the universe is infinite, for example. That's not to say that there is no longer any role for speculative philosophy, just that it's not the same role as it played in the 18th century.

"...much of speculative philosophy's ponderings have become obsolete..." Or have they? I guess that goes to the heart of what I was getting at in my question, which is, have developments in science and philosophy since Kant's time in any way invalidated his basic premise? In fact, you could argue, on the contrary, that science actually lends support to some of his ideas.
It would seem that we have reached a point where if a question can't be answered scientifically then it's assumed to be irrelevant. This generally equates to a form of realism or materialism but the ultimate basis of these worldviews is also metaphysical.

Actually, I very much agree with you. But one time on a different forum, I said something to the effect that at the frontiers of science, they're doing philosophy, in particular, metaphysics. But metaphysics by the philosophical definition, not the vernacular connotation of "woo." A lot of people there refused to consider the word in its philsophical context and accused me of saying that science is woo. A shitstorm ensued and I gave up, left the forum.

Since then, I've been very careful in how I talk about it. In the sense that scientists are trying to figure out what really is true about the universe, yeah, they're doing metaphysics, whether its ontology or epistemology or whatever. If they take the models they construct to be representative of how things really are, they're making metaphysical assertions, and when they propose such things as multiverses, etc, that we can't test, they're doing speculative metaphysics, aided by complex mathematics. But that's nothing whatsoever similar to what Rupert Sheldrake is going on about, and I never intended to imply such.

No meat...just package. Well when something more interesting then "we know that we don't know" comes up for comment I will be back. In the least people try to speculate about what would make sense as I dare and do. Philosophy like this is just plain boring defensive crap...tender citations here n there some history and no meat whatsoever... take the freaking gloves out and get into the mud for a change...this is the web.
(or go do some poetry and gardening elsewhere)

No, not at all. Kant's relied on noumena as a way to explain the persistence of reality that did not rely on the existence of an omnivoyant being - thus refuting Berkeley's idealism.

Right now with your one liners obvious remarks you are looking very much like a traffic cop writing traffic tickets !
You strike gold with your signature on top of it !

You wrote

You are correct about the irrelevance point but I think this equates to pragmatism. Reference to Rorty yields the pragmatist view that the realism/anti-realism debate in science is futile.

Yes, the line between science and philosophy does begin to blur at the frontiers of science. And yet there are some eminent scientists who go so far as to deny that philosophy is even a legitimate discipline and basically see it as theology in disguise. That seems to me a ridiculous and philosophically naive attitude. I think such people are scared by the idea of philosophy in the same way that some religious people feel threatened by science.

Both Neil deGrasse Tyson and Richard Feynman made derogatory comments about philosophy that, to me, just showed that they didn't understand what philosophy is. On the opposite end of the spectrum is Niels Bohr, who did.