Network hashrate dropped considerably, primarily due to a huge drop from ASICMiner (see: http://www.asicminercharts.com/).

It is silly to assume that the current drop is due to GPU miners shutting their rigs down. If that would happen, we would see a very gradual decrease as there is no secret conspiracy to shut down GPU mining en-masse at the same data. You'd see areas with high electricity costs shut down first, followed by more and more areas with each difficulty increase. There haven't been these huge drops in network hashrate with previous difficulty adjustments.