I believe a lot of the difficulty increase is caused by the increased prevalence of ASIC miners. Hopefully the difficulty increase rate will slow down as GPU/FGPA miners pull out because they can no longer maintain a positive ROI.
I would be surprised if there are any GPU miners left. A top-of-the-line GPU won't even earn 0.1 BTC this month, and that won't cover the cost of electricity.
I think the difficulty will continue to climb at a high exponential rate until the cost of power becomes a significant factor and that will happen within 6 months.