I’ve been running an iPhone 4 with iOS 4.0.0 and 4.0.1 on the T-Mobile USA network for about 2.5 weeks now so thought I’d give an update:

Here are the steps so far:
1. Had to cut down my mini-sim to micro-sim size. That was not difficult. If there is interest, I’ll post the pictures from the process. Took about 20-25 minutes to get it to the correct dimensions, primarily shaving off the last little slivers to make it fit snuggly.
2. Had to get it from Europe (UK) so it was factory unlocked. Not difficult, but not subsidized. However, if you think you spend $70-100 more per month on the plan (about $154 from AT&T to get a plan close to what I have with T-Mobile) it does not take long to make up the difference. And you end up with an unlocked phone so you can use it anywhere.
3. Works nicely except only on EDGE, not 3G – presumably due to the frequency differences. In theory, it should be relatively easy for Apple to open up the new frequency (e.g. no additional antenna/chip needed etc, just tuning to another frequency like a regular radio). Hopefully the chips support it, if Apple wanted to support T-Mobile and it didn’t violate their exclusivity with AT&T.
4. No visual voicemail. Not a big deal so far for me.
5. T-Mobile on iPhone 3GS (on 3.x and 4.0.0) was having trouble receiving MMS, but sending okay (e.g. Google “the media content was not included due to a picture resolution or message size restriction”). Not sure if iPhone 4 running 4.01 is still doing it.
6. Since it uses EDGE and not 3G, I can’t using internet/mail etc while on the phone. Not a big deal for me.
7. Gets equivalent or better reception in the areas (Florida only so far) I use it with iPhone 3GS’s on AT&T – e.g. same areas, bars etc. (EDGE vs 3G of course).
8. iPhone 4 has performed equal or better in reception and dropped calls in the 2.5 weeks I’ve been running it on T-Mobile US as compared to previously with iPhone 3GS.

All in all, it (and 3GS and 3G) have worked great on T-Mobile USA with the exception of 3G vs EDGE so far.