Our two neutered males didn't spray inside at all until we got a new cat last year. We've always discovered it after the fact, and weren't sure who the culprit was. In any event, all three cats get along pretty well now, so it was baffling. I researched this behavior at the Scoop on Pee & Poop website http://poop-pee.homepage.com and it's much easier to deal with when you understand cats' reasons for spraying. Last week I bought some Feliway at the vet's, which seems to be highly recommended to not just deter spraying, but to reduce the reasons for doing it.

Good luck! After my new 19" monitor was christened, I knew I had to do something.