K&N filters (if OE replacement) will not cause the warrenty to void! It states that in there Catalogs and web site.
IIRC, there is also a federal law (Magnuson-Moss Warranty Act) that states for filters, that if the dealer or MFG will not supply you with filters free of charge, they cannot void your warranty.
So if you have and OE replacement filter, then you should be ok.
But if you ahve a MAF adapter that is not Carb legal (k&N, Injen and I believe weapon R are carb legal for example) then your SOL on the warranty.
Roz