If you want to try out the 3G network, you can tether the new SE TM506 that T-Mobile started selling last week and return it before the trial period ends. Alternatively get a Stelera Option GT data card, which is compatible with the T-Mobile network; it’s how people in NYC have been using the T-Mo 3G network for months now. These are in (relatively) high demand, so you should be able to sell it when you’re done for a decent price.

//jealous because I live in SF