Looks like, at least at the beginning, there’s no chance of getting android for any other network. Here’s a review of the phone.
However, it seems that Google and T-Mobile might not be as stingy as ATT. It will accept software submissions that “don’t serve the corporate interests.” So possibly, in some distant future, they might let it run on competing networks. Or Androis and iPhone will negotiate an agreement for cross-marketing.
Of course, someone will come up with a hack, just like there is for the iPhone. You’ll be able to modify your phone to use any network. Though why you’d want to use ATT is beyond me.