A couple sites appear to be around 15%. I was hoping it would be less and with Antminer S2's hitting the market I don';t think this will change unless people stop hashing with old miners. opinions.
https://bitcoinwisdom.com/bitcoin/difficultyI was looking at this a couple of weeks ago and for most of the last year the average hashing rate increase has held surprisingly steady. This rate has slowed a little in the last two months but I suspect that all of the 28 nm ASIC designs will actually see things jump back up. As an example, over the last few days it's pretty clear that some significant new capacity has hit the system.
http://hashingit.com/5-the-bitcoin-runaway-mine-trainThe growth rate has not been at all steady. The graph on that block post is very misleading. Here is a better one:
