it's all about the towers. For T-Mobile USA you need a phone which supports BOTH 1700 & 2100. The MI-One doesn't support both. I'm going to give a t-Mobile prepay card a shot for one month. I'm going to guess that I will get 3G from 1700 towers but not be able to see 2100 towers. My reception will most likely be shitty. I'll report real-world results from Los Angeles, USA
i got a t-mobile sim. i haven't been trying too hard but it seems it doesn't work. I had a prepaid ATT sim card in there a while ago. that worked alright. i'm 90% of the time at a wifi spot so it wasn't much of a problem. as it turns out ATT stopped providing prepaid sim cards with data plans. that's when i tried a t-mobile sim and saw it fail.