Are you sure they are on? I can't see a noticeable bump in total network power. The increase is steady within normal variance. Perhaps a lot of GPU miners shut down their systems at the exact same time?
If so they have become really good at turning them on and off at the same time:
http://bitcoin.sipa.be/speed-lin-2k.pngNOW there's been a spike. Big time.
Hrm.
