I had this happen and I have tiger installed. What began as the odd freeze very soon became a total freeze and, as already mentioned, my macbook pro could only be operated by an external mouse and keyboard. Looking around at the forums I found the suspicion was it was hardware problem and particularly there was advice to depress the orange strip under the battery (hold it down with tape or something which seemed to work for people).
I was still under warranty and reluctant to use that kind of short term fix so I went to the genius bar and, after messing round with the very same orange strip to no effect they identified it as a hardware problem. They replaced the top case and keyboard. It works now, touch wood. If this happens and you're out of warranty I think it's not great you'd have to pay £130 odd for what seems to be a hardware fault giving your mbp unacceptable reliability issues. After all, these things aren't cheap and you're paying for a premium product.
The guys at the genius bar were fine, courteous and helpful and after all it got fixed. I asked if it was a common fault and the guy said it wasn't. I suppose when it happens to you it feels like a 100 % ! I suspect tho' they spend all day talking to people whose stuff is broken / not working and, in common with other customer services, they don't get paid a hell of a lot but are expected to absorb a lot of crap from unhappy mac owners and give a high level of expertise and service.