Thanks for the follow-up, USMCSS300. After your reply, I rechecked with my local dealer and they are now saying it is an Interference motor. I also called Lexus dealers in two other states and they, too, said Interference.
So now I'm a little leery about driving the car since I purchased it in September 2000, just after they came out. The IS has not been exposed to long periods of outdoor temperature extremes, and the drive belt still looks good. So I guess I just need to bite the bullet and at the very least, have the dealer open things up and at inspect the timing belt and go with what they recommend.