When I decided on a whim last Saturday to buy an iPhone - my main impetus, and I'm only half kidding here, was being able to find public restrooms for my mother during our Seattle trip ("Travel while your bladder is young, Lydia") - I still hadn't found an all-encompassing tutorial on how to do it. I figured worst came to worst I'd just resell, but I was pleasantly surprised on how easy it is. Here is the sort of list I would have liked to find after Googling "iphone tmobile jailbreak":

Step 1: Get an iPhone. I bought a used 2G 8GB model off Craigslist because I'd read that the 3G models are less compatible with the TMobile network.

Step 2: Download Pwnage Tool 3.0 and install on your computer. Your iPhone doesn't need to be attached yet, as the first part is just creating a custom .ipsw (iPhone software) file.

Step 3: Follow these instructions at Lifehacker; there are multiple other how-tos available in case you want more or less detail. Be sure that your generations and versions match up - iPhone 2G isn't the same as the 2.0 .ipsw - and that you read all the disclaimers. It took me multiple tries, probably because I was rather frustrated that I'd unjailbroken it within 24 hours of purchasing (note: you can't just click "Sure, update me with your proprietary software" in iTunes once you've jailbroken your phone. Bad things will happen). There was even a scary iPhone screen of death that looks like this:

that won't even let you power the phone down. The only way I found to beat it is to completely reinstall the regular 3.0 .ipsw file from Apple via iTunes; that should reset your phone back to its happy little "I hate this unrecognizeable SIM card, get it out of me" state, after which you can start Pwnage Tooling again.

Speaking of the SIM card, I tried the unlock/jailbreaking with it both in and out - I couldn't find an authoritative stance on the issue - and the time it worked it was in. (The SIM card cradle is at the top of the phone and can be ejected by firmly pushing a paper clip into the tiny hole.)

Pwnage Tool also installs Cydia, which is vaguely described as allowing the user "to browse and download applications usable on a jailbroken iPhone". As with any slightly sketchy endeavor, there are going to be small odd things you have to deal with - in addition to Cydia, there was a time where only 15 of the 16 icon spaces were taken on my iPhone desktop (despite there being a second page of apps).

Once you can see Pwnage Tool's funky pineapple, you should be good to go in terms of phone calls.
Step 4: Now you have to get a data plan if you don't already; 611 puts you in touch with customer service. My specific person was sort of a dud - she added on extra messaging even though the data plan covers it, and then neglected to tell me the next important step - which kills me, because TMobile generally has phenomenal service. This was evidenced by my second call to check up on my non-functioning telephoneinternet. I was forwarded to the tech department, where my new best friend David knocked the extra messaging plan off my bill and helped me with the last step, which is:

Step 5: Go to Settings > General > Network and turn on Data Roaming. Then tap on "Cellular Data Network" right above and in the APN (Access Point Name) field enter "internet2.voicestream.com" and save. Note: this is what I was told to enter; for all I know, you may need to call and get a completely different APN address from your own David. The important thing is to realize this is a necessary step, because not all the regular reps are aware of it.

Finally, I'd like to point out that TMobile was completely professional and friendly about the whole shenanigan every time I called to essentially get their help in hacking a competitor's phone. They made sure I knew that there might be instances in which they couldn't support me, but multiple reports - the one month credit when voicemail went wonky, the leaking of 3G specs - show that they're invested in making it work, which is yet another reason I'm glad I'm sticking with them.