This is an idea that I have been thinking about for a while and that is the concept of instant message filtering.
I don't get too much spam on my instant messenger (I use or have used ICQ, Yahoo IM, Windows/MSN/Live Messenger, AIM, gaim, and Office Communicator). I think I can recall only a handful of times where I was spammed by somebody I didn't know while using my instant messenger.
Would instant messenging services require spam filtering (or rather, spim filtering)? IM is different than email, you usually only want to IM people that you already know. An IM program could simply allow default access to approved people on one's contact list. In order to IM somebody you'd first have to be allowed onto their list. This is a feature that is already contained in most messenger programs.
I would think that content filtering would be quite difficult because instant messenging uses a lot of slang and abbreviations. I think the challenge would be to integrate a lot of blacklist and reputation based filtering in order to effectively meet the challenge of IM filtering. Of course, IM filtering would first have to show a need. Is it there yet? I don't know, but probably not.