You can get "unlimited" data-only service from T-Mobile for $40 a month, with no contract (getting it with a contract is more expensive, for some reason), and just purchase a bare SIM as your "phone" for $10 (free if purchased online).

You can still do voice calling with it; it just doesn't come with any free minutes. For me, I tend to use something more like two hours of voice minutes a year, so the outrageous 45¢/min usage fee is not a big deal. But even if you use 120 minutes, that's still only an additional $54 a month, which, added to the $40 for the data service, is still cheaper than the $110-$140 for the "normal" service from T-Mobile, and competitive with Verizon's $80.

Of course, you still have to have an unlocked phone, which is not exactly the cheapest option. But if you can keep your voice usage well under 2 hours a month, you'd probably make it back pretty quickly. Also, T-Mobile uses a frequency band that no one else in the world uses, so you won't be able to get 3G data anywhere else. However, both voice and 2G (Edge) work just fine, based on my experience with my N1 on AT&T's network.