Eli, according to the reports from T-mobile management during the press release, the G1 will be locked as all new GSM handsets are, however they won't be treating this any differently from any other T-mobile phones as far as unlocking policy. In other words, all you have to do is ask them nicely for the unlock code, and they will gladly give it to you (he actually admitted this in a candid interview with Engadget Mobile).
And yes, Android is not limited to T-mobile, the first device just happens to be.
The whole idea is to have an open platform, and all the major manufacturers and carriers have joined the Open Handset Alliance that Google started. Its sorta the anti-Apple iPhone business model.
Samsung, Motorola, and HTC have all expressed plans for future devices, and I'm fairly certain AT&T is on the list somewhere, although probably not until 2009. I'd wait it out for apps like exchange sync to mature anyway, and I'm sure by the time it hits AT&T, it'll be there.