The '99 I just bought had a MAF failure at only 35,000 miles (Sept '08) - car has had a K&N in it from the start. It was cleaned and re-oiled at 30,000 last year at 30,000, per the dealer notes. The first thing I'm doing is putting in a fresh paper filter. From the many used oil analyses I've done on various customer cars, I've found that silicon levels (dirt, contamination) are about double for engines running a K&N filter. The only cleanable air filter I have seen that doesn't allow more contaminants in are the oiled foam filters - there are a few out there. For reference, here's just one comparo of foam versus k&n:
http://www.smartsynthetics.com/articles/amsoil_vs_kn_two_stage_air_filter.htm
Sorry to post an Amsoil link, but it's the first one I found in Google.
The best foam air filters are those made by ITG:
http://www.itgairfilters.com/
Another good link to read up on air filter testing is:
http://www.bobistheoilguy.com/airfilter/airtest1.htm
K&N does offer foam pre-filters for some filter types, which would bring their performance up to par with other filters. But just changing your paper filter more often will yield roughly the same performance (at a greater cost of course over the lifetime of the car if you don't count MAF failures with over-oiled K&Ns).