When I got my iPhone a couple of weeks ago, I went into the local TMo store to change my Sidekick plan to TMo Total Internet. I told them I was getting an unlocked iPhone. They have no issues with that.
The TMo guy said it might take as long as 48 hours to transfer. When the 48 hours were up and I still didn't have data, I went back. The guy checked that the plan had been changed, and then called TMo tech support and set up my iPhone for me. Granted, it was dead simple once he got the right guy on the phone, and I could have figured it out myself by reading this forum--but I was impressed by the customer service.
And Edge on my 2G iPhone is usually way faster than it ever was on my Sidekick II. I wonder why?