Well, so far so good. After a long download (4.3GB!) and a quick install, Mountain Lion runs great on my MacBook Air, and so does PI (1.07.06.0793 64bit) with all updates installed.
I loaded an existing project, and everything looks good. Once I have a little more confidence in ML, I'll install it on my workhorse iMac and do some serious PI processing. I'll let you know if I see any issues.
Sean