If you feel so compelled to, I think you might be able to flash it to Virgin Mobile, I'm not sure though. And you wouldn't get 4G.
Another thing to think about is that Sprint's is the only one that is for sure coming out soon. There has been nothing from AT&T or T-Mobile about their Galaxy S II variants being launched any time soon.
yea go for att or tmo much more fflexibility with gsm. Get the one that has the same bands as the carrier u will go to. Also I know there is a euro version whichh works on att so u can check ebay for that and no need to wait for it to launch here.