as jonnyxedge noted, TMo is excellent at warranty coverage - I've found it even better when purchasing the phone in person at a corporate store (as opposed to a mall franchise). Essentially, as long as you've had the phone less than a year, there are no visible cracks (e.g., from a fall) and the moisture indicator hasn't changed color, the corporate store will simply replace your phone on the spot (they've done this for me and a friend as well - took all of 10 mins).
Re: the replacement battery cover / retainer for the pearl: have you searched online? you might be able to get a replacement housing, complete pretty inexpensively by now...