The first thing I'd try to do is to determine whether or not the squeal is caused by the belt or by something the belt is driving. Is the belt tight and running true? An easy way to check for belt squeals is to get some brake fluid, and either brush or spray it on the belt while it's turning. It won't harm the belt and will wear right off, but if it changes the sound, you need to look either for slippage or something that is causing bad belt tracking.
The idler that holds the fan is one possibility, though it rarely goes bad. The water pump may or may not leak when it goes bad, but it will usually rattle and develop play which you can feel. However if it is wobbling, it can also cause the belt to squeak. Just tonight I replaced the PS pump on my 99, which was squeaking. This was really easy to detect with the belt off, since the PS pump should have no roughness or noise at all. I thought it might be the harmonic balancer at first, so I got underneath and squirted WD-40 between it and the timing cover, figuring it might change the sound. There was no change, so I eliminated that as a likely squeak. I also replaced a slightly wobbly fan bearing, bracket and all, since I was doing the job anyway and the junkyard had one for $40. Alternator bearings can also start squeaking, and may do so long before the alternator actually starts to wobble or otherwise misbehave.
Though it's a drag, your best bet is probably to take off the belt again and wiggle and spin individual components to try to find one with play or roughness.
