How can I recover when my iPhone isn't responding to touch when using Guided Access on iOS 6?
Seems like a solution is to hold home button + volume down + screen lock until device forcibly reboots.  After that, guided access began responding properly again and I was able to exit the app.  

Edit:  Through trial and error, I felt this was home button + volume down + screen lock, but as comments  have noted, volume down is not necessary.