I think it is ridiculous to be required to sign a two year contract for a service without some sort of subsidized equipment. But, that's exactly what AT&T and T-Mobile will claim is necessary if you call their phone sales line. T-mobile will, however, be happy to set you up with a "pay us you go" plan and then add an unlimited data package for 19.99 a month with no contract. You will have to go to the store, but that's completely worth it. I found this out on my own, but it has been discussed in other boards and I haven't seen it here yet.
Apparently there is a similar trick that works on AT&T, which others might like better because T-Mobile's new 3g network isn't compatible with the Shift and you are stuck in EDGE. I will say however that T-Mobile's EDGE network on my shift has been much faster then AT&T's 3G service on my iphone. I also like T-Mobile's customer service better, although neither are great.