My contract is up with AT&T and my 3GS is showing it's age so there's a very high probability that I'm getting the next iPhone. I hate AT&T though, as it's pretty expensive. Right now I pay 160 bucks for two phones (one normal, one iphone). The wife pays 100 bucks on her Verizon phone. So that's $260 going out for cell phones which is crazy.
Like you I had the unlimited data plan but had to ditch it as I needed to add on tethering for my laptop (didn't jailbreak, went the official route). It seems like AT&T want to do everything they can to get rid of the unlimited data users.
Sprint has an unlimited data plan and have recently stated that it's "here to stay". So if sprint gets the phone tomorrow and the prices are still decent, then I'll cancel Verizon and AT&T and put us all on Sprint and probably save 100 bucks a month doing that.