I think you could not really pull a GPUminer on bots - that usually need drivers installed correctly etc and it really slows down system performace - I think it would slow down their computers too much so most of the victims would simply buy new hardware.

However, I'm administering a Computer pool with 80 machines here for work, and thos machines have to stay on at night but theres no one being able to use them, so what I can tell:

CPU: Intel Q6600 -> 4 MHash/s/Machine
GPU: GF 8800 GT -> 25 MHash/s/Machine

that makes roughly between 25 and 30 MHashes per machine, for me it's about 2GH/s, but of course a large scale bot network could do much better.