I'm gonna move this to the carrier comparison sub forum located in the android lounge. But as far as i know, none of the big 4 carriers allow this in the US. There might be prepaid options that may allow you to activate a smartphone with data. Hopefully at the carrier comparison forum, someone can confirm.
For postpaid, T-Mobile does if you bring your own device. If you do go that route, though, it doesn't really hurt to get a data plan since the rates on their Value Plans are so cheap.
For 500 Minutes + Unlimited Text + Unlimited Plus Data (throttled after 2GB):
T-Mobile Classic: $70/mo or $1,680 for 2 years
Galaxy S II (subsidized): $200 (after $50 rebate)
TOTAL COST: $1,880 + regulatory fees and taxes
T-Mobile Value: $50/mo or $1,200 for 2 years
Galaxy S II (retail): $550
TOTAL COST: $1,750 + regulatory fees and taxes
Data is optional on T-Mobile Value Plans so if you choose to forgo data, it's going to be:
T-Mobile Value: $40/mo or $960 for 2 years
Galaxy S II (retail): $550
TOTAL COST: $1,510 + regulatory fees and taxes
The $550 is recommended retail pricing for the S2. I think you should be able to find it for less than that. Alternately, Google is selling the Samsung Galaxy Nexus directly for $400 and it supports both AT&T and T-Mobile frequencies. That drops down the 2-yr cost for the Value Plans to $1,600 with data and $1,360 without.
If you really want to go cheap and don't make a lot of calls/messages, either AT&T or T-Mobile prepaid will also work. Just buy a phone and get a prepaid SIM card. If you're not a heavy user, cost is as low as $100/365 days on AT&T and $10/365 days on T-Mobile (after initial $100 load for Gold Rewards status). AT&T charges $0.10/min and $0.20/msg on their Pay As You Go plans while T-Mobile charges $0.10/min and $0.10/msg (with Gold Rewards).
