It is popular to target gamers and bind malware with games and cracks, especially those that are popular atm. Even if the avg. gamer gpu does 50-150 MH/s with 10k bots it would be ~1TH/s (assuming 24/7 100% use, let's say 1/3 of that is a more realistic figure).
Bitcoin botnet operators even make AMAs now:
http://www.reddit.com/r/IAmA/comments/sq7cy/iama_a_malware_coder_and_botnet_operator_ama/

I operate a ~10k botnet using a ZeuS software I modified myself, including IRC, DDoS and bitcoin mining (13GH/s - 20GH/s atm).