I did basically the same about a year ago. Bought MyTouch 4G from T-Mobile without a contract for a full price, asked them to unlock it, they didn't have any problem with that - got my information, and emailed me the code the same day. After that I just got a cheap phone with a SIM from another operator, sold that phone (brand new!
), put the card into MyTouch, and that's it!
What network does WalMart actually use? I don't think they have their own, do they? The thing is you have to be sure the phone is compatible. Most (all?) T-Mobile phones use AWS(UMTS) frequency for 3G connection. AT&T, for example, uses different frequency.