Folks might try updating their Flash Player. (a constant problem, as many of us know). I have had this freezing when in sleep mode since upgrading to Mountain Lion. Today, I checked the usages, and Adobe flash player plugin was using over 17%. I did the lastest flash install, and its using an infitisimal amount now. Only time will tell if that has solved my problem. There is obviously SOME issue here, and I agre with many posts, that its probably caused by a 3rd party plugin etc, thats running in the background. Flash is a good place to start.
For me, at least, it was 100% reproducible using nothing but OS X. Just had to use Apple's Java and run Minecraft in it, and toggle full-screen mode a few times. Then it got fixed, I think around 10.8.3 or 10.8.4, and after that I have had no problems whatsoever.
So for me, it appears to have been Apple's video driver, not any third-party plugins or anything.
Ultimately, it comes down to this:
If user code can cause the system to hang, there is a bug in the kernel. Period. It doesn't matter whether it's Java, or Flash, or whatever else; if any sequence of bytes executing as a non-root user can cause the system to hang, the kernel has failed in its primary job.
Luckily, Apple tracked down and fixed the instance that was affecting me.
I too had a similar problem that was extremely reproduceable on Mountain Lion. I have a MacBook Pro with dual NVidia and Intel Graphics GPUs. Seemed to always be related to a switch the OS was attempting to make to spin up the NVidia GPU for intensive graphics operations. The failure occured 100% of the time in Safari at a specific web site which rendered some of the content using svg tags. Also failed at times in Aperture and the Java settings applet. After working with apple tech support (they reviewed various log files, system crash reports, system info, etc) they had me send in my system to replace the motherboard. It was a faulty NVidia GPU causing all the problems that only showed up using Mountain Lion. Since the motherboard was replaced, I've not had a single hang or problem related to the NVidia GPU. If you are still under warrenty I'd suggest getting Apple tech support involved.