I just got a Verizon iPhone but I wasn't ready to get rid of my AT&T account yet. Verizon gave me a temporary number; I'll try it out for a week or two, then port my number over from AT&T if I find the coverage is decent.
I don't want to port my AT&T account over to Verizon just yet, find I don't like it, then go back to AT&T - I'll have lost my unlimited account w/AT&T in the process.