My cam belt hasn't gone, but just last week my auxiliary drive belt went. The cause of this was a worn tensioner. I say worn, but the bloke at the garage put it rather more colourfully; 'To say it was sh*gged would be paying it a compliment'. Apparently the whole assembly colapsed and then melted (or perhaps the other way round).
Anyway, apparently I was very, very lucky the strain of this happening didn't make the timing belt jump a couple of teeth, trashing my engine.
My car is a 1999 TS with 49,000 miles on it so I'd recommend changing all belts and tensioners well before that sort of milage. Its bloody expensive to do this, but not as expensive as a new engine.