What i think would be useful is if someone with the right set of skills and resources would monitor Deepbit and Slush, and figure out how much of their power comes from botnet participation...

BtcGuild was big until they supposedly decided to ditch a botnet and got DDOS'd. It will be interesting what size it will be once it comes back.
Perhaps, on basis of that, we'll be able to make some wild guesses regarding how much of Deepbit is actually botnet power being stolen from unsuspecting internet users...

Botnets simply cannot contribute that much. The average computer could only hash pathetically. Even tens of thousands of average computers would probably only be 50-100 gh/s.