I've been with AT&T for years, but with a plain old phone, therefore no data plan. I had, but returned, an Atrix 4G, and while I had it I fell in love with Android. I have been doing a lot of research on various 4G phones / plans, and what I really want is an Inspire 4G, but on T-Mobile instead of AT&T. I can get a practically new Inspire 4G locally for $350. A T-Mobile sales dude told me "any AT&T phone can be used on T-Mobile." If I buy the Inspire 4G and root it will it work if I switch to T-Mobile? Or was the sales dude naive / misinformed / lying?