Some belts stretch in the first week or so. I'd imagine most mechanics overtighten the belts slightly (causing squeak) because they assume you won't go out and redo it when it stretches (and it is a rather pain to do, unless you are familiar with it, especially if the adjusters are stripped, which they probably all are after 16+ years).
If it doesn't go away, then you may want to call or stop in and have them look at the belt tension. They should do that for free, since they installed the belts. In fact, you can probably do it now.
-Guy