I totally agree with Rockstar. I've got 50K on my s4 and I plan to change the belt by 60K. Having said that I'm not sure when I should. I'm simply assuming based on nearly every other car I've owned has been 60K! I'd better check this. At the mo I'm undecided whether I'll do this myself since I now have the Erwin manuals. It will depend on the quoted cost.
My mate had the timing belt go on his 2.8 V6 quattro. He had to replace the valves, the guides the cams, head skim. All in, I think he was £1200 and that was cheap because he got a local £30/hr garage to fix it!
I see VAGPARTS sell the RS4 heads for £5K. I bet the S4's are not much cheaper, and that's before labour!