Well, the mystery may be solved.

My local mechanic took a closer look and said there are nicks in some of the pulleys - looks like some rocks or something got kicked up in there and damaged the pulleys. The belts are actually getting worn prematurely as a result.

He said I'd have to check with the dealer if I want the pulleys replaced, and seemed to indicate it might be rather expensive.

In the mean time, they replaced the belt under warranty again for about the 3rd or 4th time. It kinda surprises me that they keep changing the belt for no charge even though it's obviously not a problem with their work/materials. I guess they are being nice as I'm a regular customer.. Hell, as long as they keep doing that I'll give them all my business.

So - guess I'll just replace the belt at every oil change or two.