Sprint has an unlimited data plan for $100 bucks a month, it's a bit pricey but they do have a great selection of smartphones to choose from. One of the cheapest unlimited data plans I've come across would have to be Boost Mobile's $50 a month data plan. Boost Mobile doesn't have a great selection of smartphones but they did just recently get in the Samsung Prevail running Android 2.2, and of course it's contract free!
Here's a copy and paste from Boost Mobile's site, hope all this helps.
Boost Mobile® Monthly Unlimited is just $50/month for unlimited nationwide talk, text, web, 411, IM & email with no contracts. Voicemail, long distance and call waiting included. All backed by a dependable nationwide network. Activate on Monthly Unlimited with Shrinkage and shrink from $50 to as low as $35/month.