I've been having the same problem with my Jeep. After I changed the belt it will squeal like crazy when I first turn the a/c on. I had put a new Goodyear gaterback belt on too. I changed it out with a Dayco and while not as bad, it still squeals sometimes. It is so loud it scares drivers that are beside me.
Funny thing is I NEVER had this problem until I changed the belt. If anything the belt is slightly tighter than it should be. One thing I did find though is the a/c was overcharged, and I think there was some liquid freon getting back to the compresser.