I've done this with three phones and nope- nothing to worry about! Updates and the like are all easy peasy and there's no way for you to lose your unlock. Tell your girlfriend enjoy! :]
Nothing to worry about. Took an iPhone 4 on 4.3.1 on a gevey unlock, factory unlocked it and updates to iOS 5.1. No 3G will work, edge is all you can get from tmobile, they utilize a different frequency for their 3G service.