odd, I was just going to post about this... my Patches has it too, and the vet said it can happen in cats that are heavy, they are not able to "bathe" themselves everywhere, and that sure is true in his case! I going to give him a bath tomorrow with someones help. that and brushing is all you can do.
is your cat overweight? if not then I don't know what casued it, you should ask your vet.