You've probably already read this, but Microsoft recently announced the acquisition of Sybari. This company makes anti-virus and anti-spam products. As of right now, Sybari focuses on AV for email and instant messaging systems; it does not run on desktops or on servers.
There's no telling exactly what MS is planning to do with this acquisition. They (we) could continue to market the software as a part of Exchange (for email) and Live Communication Server (for IM), or possibly adapt the virus scanning technology to run on the desktop. Someone from MS may have already said something about this, but I don't know.
I attended a LiveMeeting last week with Dave Thompson and he implied that the Sybari acquisition would feature in message hygeine tools/UI in Exchange 12