I have the exact same problem. Mine is an '03 with only 5,000 miles... not sure what is causing it other than just the belt is cold and slipping a bit until it gets warmed up. Mine was squeaking something awful at one point, no matter how long the car had been running, but the dealer fixed it and it went away... then gradually came back as this "cold squeak" I'm experiencing now. To me, it hasn't been a big enough deal to take back to the dealer (yet...).
I know other '03 owners have experienced the same thing...