T-Mobile already has droid phones. I bought an HTC Magic with android 1.6 from craigslist. The phone WILL NOT operate without a sim card. On bootup, it just sits and says no sim card. The only thing you can do is dial 911. I went to t-mobile, and they gave me the same as verizon. I had to get the most expensive plan they could dream up. I paid $60.00 to get a SIM card and now I have a droid phone and no lock-in. Since I didn't get any subsidized phone from them, I have no contract. Now from here you can do a couple things.
1. You can get an unlock code. This apparently removes the t-mobile lock-in. I've never don't it, but I'm almost certainly going to try it.
2. You can "root" it. This allows you to take any droid image and flash in onto the phone. I've have absolutely no experience with doing this, but there are articles on the web with detailed instructions.
* I don't know if 1 is required to do 2.
If you want to phone for hacking, I just laid out the plan for you. Buy one used from ebay or craigslist or wherever, get a month-to-month SIM card (if you don't already have one). and you're off and running.
One more thing. All these posts about how verizon sucks with their cancellation fees, nickel-and-diming you for everything, making it impossible to figure out what you're going to be charged for is exactly what these people want to turn the web into.