Yeah.. I also had a bad day with this..
I have the unibody Macbook, I love the thing, but.. with 1g products there are sometimes faults. Now.. When this happened to me, I didn't hear anything loud enough to make me jump up and stop what I was doing, but my fan, ever since I purchased my Mac, was a little.... loud. But.. me being me just ignored it, didn't have time to bring it and get it diagnosed. About a week later, BAM! 10.6.1 locked up. Forced a restart, and the same thing happened to me, everything your experiencing.
After this, I sent my product back to Mac, They mailed me a box over night, and I have Fedex pick it up same day. A day later they called me, and told me that my one year limited mac warranty will cover my damage.
They replaced the following faulty parts.
-Internal HD
-Internal fans
Judging by what your going through, you should do the same.
But one way to be sure, if your comfortable and ready to admit you have a problem, when you CAN get it to a start up disk. Use the Disk Utility. and do a complete format of your internal, but use a high security raiting that way you get the best test. In most cases, if you have a faulty HD you'll get a very depressing message.. "Could not finish formatting".
Well, there's my two cents. Sorry if someone else has already answered your question, i kinda just skipped to the bottom =)
But lemme know if this helped at all.
Edit-
And for the record, don't start tilting the laptop and hoping the disk will fall out. my god...