I got my t68i last year through Amazon with T-Mobile service. I'm sure you're aware, but those rebates take a while to get back to you...but back to your question - I think you'll be fine. All Amazon seems to care about is that you activated your service with T-Mobile, and as long as your rebate form includes some sort of documentation to that effect, you should be okay.
This is just a guess, though. I really don't know for sure.