GOOD NEWS: I think the iPhone 3G will be hacked... just have to wait a few weeks after July 11th. They have not failed us yet!
BAD NEWS: There is that annoying in-store contract/activation hurdle, since it won't be as easy as walking into an Apple Store and picking one up. I'm predicting the adoption of the BUY, SIGN, WAIT, PAY method where you buy a new iPhone 3G, sign the contract, wait 30 days then cancel and pay the termination fee/probably the first month of service (getting a bit pricy there!). If this proves to be successful, think of how many contract cancellations there will be!
So once you finally have your unlocked iPhone 3G running on T-Mobile, you will still have to wait who knows how long for TMO to roll out their full 3G service (I believe its only VOICE now). Will it work? I'm hearing a lot of YES and NOs. Will your current data plan be compatible with the 3G? Too far in the future to tell.
In the meantime, I'm gonna be looking for a 2G iPhone