@Setanta,
Rats.
I use "So" and "Okay, so," all the time because...erm...I think it makes me sound colloquial.
(I'm not kidding, I write a monthly letter to the shareholders of my building and there's always a couple of "Okay. So"s in there and an ...erm..or two.)
I never use it/them when I'm being serious or speaking officially. The producers of the radio news show ought to have a word with the reporters.
So, not colloquial, more like juvenile.
Joe(double rats)Nation