The 3GS was going to be $399 since I had bought a subsidized Curve for like $199 with a $100 rebate or something like that. But now, according to att.com, the phone showing on my account is a 2g iPhone, when I am really just 12 months into a Curve contract.
Well, I have no real explanation of that. But then again, I'm not into trying to find a work around for the rules. If it worked congrats, but you may be the exception rather than the rule.
Also, becareful AT&T doesn't charge your account the prorated amount of the curve on your next statement. They probably won't, but I'd set that money aside just in case.