Well, for what it's worth:
I have a '98 BMW 740iL with Nav system that did the same thing. Usually, if the unit can't read the CDROM (or DVD), it means that either the disk is scratched, or the head is dirty.
So I went out and got one of those "head cleaning" CDROMS from BestBuy, and ran it a couple of times. No more problems!
I'm sure the purists would disagree on this method (it might scratch the head--the cleaning CDROMs have these small bristles sticking out of them, which swipes past the head as it rotates, thereby cleaning them). But hey, it sure beat paying the dealer big $$$.