A possible solution? --
I think I had the same problem as many (if not all) of you. I replaced my stock MBP hard drive with a Western Digital Scorpio 320 GB when I bought the computer. Working fine until I installed the EFI Firmware Update 1.7 two days ago. The update completed, but upon restarting, the computer would hang at a grey screen (no apple, no spinner). The drive worked fine in an external USB enclosure, and I could re-install my original stock hard drive with no issue in the MBP. After a support call to Apple and an e-mail to Western Digital, WD came up with a solution that worked for me. They said "It appears the firmware update damages the boot sector on the drive. You can always try Disk Utilities to see if you can repair it, if not then you may need to reinstall your OS on the drive."
This was a problem because I could not boot from CD when the "bad" WD drive was installed in the MBP, nor could I access open firmware, do a PRAM reset, or any of that stuff. It would just hang, similar to what happens on a PC when you set the jumpers wrong on an IDE hard drive. What did work for me was the following:
1. Removed third-party hard drive from the MacBook Pro.
2. Installed third-party hard drive in a USB enclosure.
3. Using a working Mac (Mac #2), I plugged in the USB enclosure and ran "Repair Disk" in Disk Utility.
4. Disk Utility reports "Repairing Disk..." and it takes a couple minutes.
5. Removed the drive from Mac #2 and plugged it into the MacBook Pro.
6. Booted up MacBook Pro from USB enclosure to make sure it works (for testing only).
And it did boot! Earlier, I tried this procedure using the OS X restore disc on the MBP with no success. I'm guessing Western Digital is right about the boot sector on the drive. Disk repair fixed it, but if it hadn't, I would have tried repartitioning and reformatting next.
I have yet to install this back in my MBP, but if you don't see another post from me you can assume I got it working.
Hope that helps some of you. It's been a frustrating 2 days.