[Short Version: Try a Disk Utility (Repair Disk) and (Repair Disk Permissions) on the boot drive]
I don't know if this is going to help, I stumbled onto this thread looking for something else, but here goes:
I think I may have experienced this issue or something similar last month. I got a new mid-2010 MBP 17" stock with the matte screen from my local mostly-friendly Apple store. After doing a user/etc migration from my 13" MBP (late 2008) I replaced the 500GB HDD with a disk utility cloned 256GB SDD (Patriot Warp v3, saving for something better!) and replaced the RAM with two 4GB Transcend modules the day I got it. About two weeks after that I had a couple of freezes similar to what's been described here. gfxCardStatus didn't exist for me at the time so I had no idea which mode I was in most of the time, although I had verified switching with System Profiler. Then I had another freeze. This time on powering back on I had the grey "can't find a boot device" screen. Odd. I thought maybe I'd not secured the SATA cable or properly seated a SODIMM or something, so open up, check, and re-seat, no dice. On holding down the option key the drive appeared, but wouldn't boot. Since owning a MacBook I've learned a few things and combined with my well honed paranoia: I keep a Carbon Copy Cloner'ed copy of the drive (updated about once a week) on the original HDD in a USB/eSATA enclosure. (Love using the 34 slot with an eSATA card for CCC BTW,
WAY faster than USB or FW) So I booted off the USB drive and did what's become a universal troubleshooting step for me when anything weird rears it's head with Snow Leopard: Run Disk Utility and do a "Repair Disk" and a "Repair Disk Permissions" on the drive. (Yeah, I could have used the install DVD for that too {which I have copied to an 8GB USB key - much easier to carry around}, but I was getting ready for more in-depth diagnostics.)
There were a few minor errors repaired that I didn't note down and didn't seem anything important, but the drive now booted right up and since then (touch wood) I haven't had another freeze.
I can only think of some very convoluted reasons why such a simple fix would solve such a bizarre, apparently hardware related problem, but that's what happened. It's probably worth a try and YMMV.
(I'm reserving the chicken sacrifice and the genuflecting to Cupertino for when ** doesn't work! 🙂 )
James