Re: Blackberry, Sidekick, ????
Sheesh, I know more about PDAs and Smartphones than I should, as I've owned the top of every line and go through them like underwear. Here's my quick take:
Your real cost is in your service plan. So you should be looking at internet rates. If you only really will use email (not browsing and multimedia) on the internet you could be able to get away with some of the starter data packages, but I recommend finding the cheapest unlimited package.
One thing that can possibly help reduce your costs is Wifi support on the device. If you are mainly going to use it at home or at places with Wifi you can access you might be able to get away with a smaller data plan or none altogether.
If you want to use the phone for anything else the software platform is important. To make it real simple the mobile platforms I recommend are Apple's iPhone and Windows Mobile.
Apple's usability and integration of it's apps are almost enough to make me adopt it despite it's horridly proprietary nature. I'd have to hack the hell out of it to even use it (it's not available where I live) but it's almost good enough to make me do so. But the downside for you would be that a lot of its strengths lie is the multimedia capabilities that I'm not sure you'd use. However it's usability breakthroughs are very very significant and much other mobile experiences are not nearly as good. If you are comfortable with technology it might not be as big of an issue but even then it will be a more enjoyable experience than any other smartphone.
Then there's Windows Mobile and the Pocket PC Phone edition platforms. I currently use this platform because it's open enough that I can install the various software I need and want. I can use it as a bluetooth modem for my laptops when I'm away from home, I have gigs of music I stream wirelessly to my headphones and all sorts of geeky stuff.
And a quick note on the blackberry: I don't like it because it can't do much of what I need, all it's hardware is too bulky for me (I like RAZR thin or I wouldn't use it as much) and because the biggest selling point ("push" email) is something I don't care enough to replicate on my phones even though I can. But I will say that the hardware usability and email implementation has been the gold standard in mobile email. Nothing else out there really has this kind of usability when focusing just on email.
Here are some strictly hardware recommendations, the real cost is in the service plan you can get away with.
T-Mobile Dash - This is like the BlackJack and Motorola Q in most ways except it also has wifi. I'd use it if it weren't so damn ugly or if I needed wifi.
All things considered this phone is my recommendation to you.
Here is a link to the phone
The reason I recommend it is because it's a nice thin phone with a fairly good keyboard, wifi and most of all because it looks like T-Mobile offers a data-only plan that might well be the cheapest choice you have stateside for your intended use. You can also probably find it cheaper from someone else (if you do, note that the real name for the device is HTC Excalibur and it's a T-Mobile exclusive in the US under the "Dash" name).
T-Mobile's Data only plan
I'm not sure that you can easily find unlimited data in the US for under $60/month so this is the best I think you can find.
Here are some other phones I recommend but they don't seem to fit your requirements:
Samsung Blackjack - Can be found as low as $75 with a plan. This is my primary phone right now. It's very thin, less wide than the Motorola Q it's copying and runs on GSM so I can swap out my SIM cards and use it overseas. No Wifi here.
iPhone - The proprietary Apple/Cingular nonsense is what keeps me from using this. Once it's a bit more network portable (i.e. the hacks stand the test of time) and I can more reliably add third party apps then I'd probably adopt this one when the mail client improves. It also supports wifi.
Motorola Q - My previous phone. Same software as the Blackjack (Windows Mobile). It has a better keyboard in my opinion than the Blackjack. No Wifi though.