My iMac 21.5" just started doing these random hangs. It only happens once or twice a week for us. I have kept OSX up to the latest and greatest versions without fail since I purchased this computer in the 2009 timeframe; I am now running 10.8.2. I have no idea if it is 10.8.2 related, or just untimely h/w malfunctions.
Within the past month, both times the system hangs have occurred while in Safari at non-descript sites (Amazon.com today); since my wife and I share this machine, we sometimes use Chrome and other times Safari. The hangs are as described here; mouse continues to update, but it is not possible to click on anything or get any reaction from the iMac. Only a hard reset with the power button gets the system to come back to life.
This feels like a hardware problem, but I admit it could certainly be a driver problem introduced in 10.8.2. Wierdly, I just received an email from Apple (my iMac is still under warranty) in the past week that suggested I update my 1TB hard drive since there are known problems with my h/w; just take it to the AppleStore and get the fix after I do a Time Machine backup. I suppose (?) it could be hard drive related, but who knows... anyway, if after I udpate the hard drive the problem goes away, I'll repost that information here then.