Still no links?
In the absence of those let's take a look at some stats. Last I looked there were around 134,000 people on the Audi R8 leaderboard alone. That's obviously not everyone who's played the demo but let's say for the sake of argument that it is. So far I've see this one report of the track vanishing - although only in replay and only on one portion of the track. Let's say there's 50 other people who have posted about this (I'm guessing a bit since I haven't been able to find other reports on this.) And let's say there's twice that many people that are having the problem and didn't bother to report it (i.e. 100 people all total). That's .074% of the users that are playing the demo that are seeing this issue. Not even a tenth of a percent. And that's assuming that the R8 leaderboard represents all the players playing the demo.
I'd say that points to hardware problems. Are you running the demo from the hard disk? How full and how old is the hard disk? If it's heavily fragmented I'd bet that that could cause this.
