I have a 2000 RX300 with about 53k miles and started experiencing the same issue about two months ago. Thinking it was the belts, I applied spray-on belt dressing which only made the squeeling worse. I took the car in for the 55k service and they ended up replacing both belts to the tune of $175. This appears to be a common problem.
Cuball, thanks for the heads up about getting them readjusted. I'll be listening...