I took my 2000 RS in for a whirring noise when under acceleration and they diagnosed it as a bad rear diff. Said they also have to replace the "drive shaft", but he might have meant half shafts the way he was talking.
I thought this was interesting as I remember people on the board saying the rear diff was pretty indestructible. The service mgr also said it was a rare thing to go out. Rare enough that the parts had to be special ordered. I will try to talk to the actual subaru tech when I bring it back in for the service. AFAIK, this will all be covered under the drivetrain warranty.
More updates when I get the diff replaced...
*edit* This is all with stock power, no rallying or tracking the car, etc.