I switched from AT&T in the US to T-Mobile in the US - and yes, just for the N900.

I don't care about the carrier (so long as they are GSM). I will go to whichever one supports the phone I want to use. Were I back in Europe I probably would not have switched carriers - unless there was a great deal to be had on voice/messaging/data.

As it turns out, I'm paying T-Mobile a little less than I did AT&T (I had an iPhone plan there before I switched to my N97 and then on to T-Mobile).