[Update] I can confirm that the iPhone is easily unlocked and very usable on T-Mobile USA. ZiPhone is your friend.
Even I'm getting sick of relentless procrastination in getting a new phone. The problem is this: I want an iPhone with 3G speed with reasonable monthly charges, and no such thing exists.
I'm thinking of going the route of getting an iPhone, unlocking it, and staying with my existing T-Mobile plan (1000min/$40 + $5 for 200 SMS messages, + $5 for T-Zones = $50). One of my main motivations is avoiding the 2 year contract. I'm going to use this post to gather my notes, and if you have experience with this please comment below.
It appears jail-breaking and unlocking the iPhone is doable, and the chances of bricking it are low. Here's a sample thread on this topic.
I'm not sure if T-Zones is enough to get the iPhone web stuff going. Some claim it is. I'll need to dig into this more.
Visual voicemail is said to not work; that's not a big deal for me.
If you have first-hand experience with unlocking and using with T-Mobile (or other carriers) let me know.