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.
Note that the spike goes in the wrong direction when the FPGA clusters "just went online", and also compare with this
historical chart which shows the variance on a longer time scale.
Yup, I'm aware of both of these things. Just pointing out that the hashrate has gone from "below the difficulty" to "double the difficulty" in the past several hours. This may or may not be significant, but it's at least eyebrow-raising.
EDIT: It looks like it's peaked and dropped down closer to the trendline now, though.
