Similar issue here. 2011 13" MBA i7 256Gb SSD. It will just freeze completely--no warning. I suspect that it may be due to the OS losing a connection to the KB and trackpad. I say this because once I was able to press the power button and I did get the "What do you want to do?"-type dialog--which of course I could not answer because I had lost input capability. However, the next time it happened even the power button did not cause a response.
So far this has happened 3 times in the past 2 or 3 days. Interestingly, I have had the MBA for a couple of weeks now and didn't notice it before a few days ago. The one change I made in the past few days that could have done something "dangerous" (potentially) is that I installed the latest version of Parallels. Anyone else load Parallels before noticing this?
This is frustrating. I have not seen such "total freeze" behavior at the OS level since Win NT.