Useful details: Core2Duo mid 2010 Macbook Air, 128Gb SSD, 2Gb RAM.
Since I upgraded to Mountain Lion I've had this occur a few times. I'll be running my system and it'll completely freeze, no response to keyboard or trackpad other than the ability to hard power off.
In general it's when I have VirtualBox, Safari, Sublime Text 2, Terminal and Spotify running, but it'll be fine up until some unknown point and then thats it. Occasionally it'll unfreeze and I'll have messages about UI updates being suspended by (insert app here), but most of the time it forces the fans on high and starts trying to crunch on something.
Mountain Lion seems a bit snappier than Snow Leopard was for me on this machine, but it seems to be a large chunk worse for memory management, has anyone run across similar issues? Starting points for me to try optimise?