Cingular already offers such service for Microsoft, AOL, Yahoo! and Google, thus there is no reason to beleive they will stop it. Plus, as fleurya mentioned, there are Microsoft-compatible programs for OSX already.
The phone is made by Apple, but Cingular is the service provider and they are going to give a package as atractive as possible, that means IM. I see no reason for Apple to block or prevent this. I would give it a 90% chance that it will have such service at launch or in the near future (after the release date).