I had a VirtualBox (Windows 7), ubuntu was the host os, window open, I had Chrome in Ubuntu open with Slacker playing music open, I had Thunderbird mail open, and Tickr running news across the bottom -- and then everything came to a crawl. The music kept repeating 2-second clips over and over again, the tickr app started stopping and starting, etc. I tried closing windows, nothing helped. In fact, it looked like chrome was unclosable.
Then, I remembered something i learned during the beta -- you could restart unity from the command prompt (Ctrl+Alt+F1 login), by running "unity --reset" (then i think it was ctrl+alt+f7 to return to window manager), I did this, and my system became responsive again after unity restarted, music started playing again, tickr started running smoothly, and the windows 7 virtual machine was responding normally..
So, can someone tell me, why does unity occasionally (not too often) cause my system become unresponsive?
I realize this could be the fault of compiz or unity, but restarting (resetting) unity seemed to be the trick to fixing it.