Personally, I don't think it makes sense for TMobile to get ifones. Their plans are $20/2GB/month the last time I check, therefore they're charging customers more upfront(e.g. $280 for SGS3 instead of $200). If they're offering ifones, then they either need to heavily subsidize it, which makes it no profit or money losing business, or charge the same as other phones(e.g. $280 for ifone 5 instead of $200 like every other carriers, apple won't let them), or the 3rd option, have a different tier of plans for iphone, that will probably complicate their current system and plans.
Also, keep in mind that, there are currently many unlocked ifone running on TMobile with 2g speed, in order for ifone to be compatiable with 3g+, they need to remap a lot of their bands.