Ditto. Very, very frustrating. I've been blaming the Windows 8 VM running in Parallels but I've actually been able to narrow it down to Flash video running in a browser.
Having said that, when flash video is played in Safari, it doesn't appear to be a problem, yet.
In Firefox or Chrome though? Youtube will eventually kick the keyboard and trackpad off, and the sound cuts off at the same time. Heck, even the embedded "live feed" on the cnn.com site crashed my inputs.
System looks like it's operating normally, you just can't type or mouse. Even unplugging the keyboard and plugging it back in doesn't work, and plugging in a mouse has no effect.
I'm running a brand new iMac 27" with a second screen (Apple monitor).
I've been a big Apple proponent for years, but if this keeps happening I cannot recommend the systems. This is actually worse than Windows in my opinion, as it's stability has been steadily improving over the years. I appreciate that Apple is trying to get new features in, but sacrificing stability is a huge mistake. I'm sure they didn't do it intentionally, but they should be moving Heaven and Earth to fix it.