So if a MacBook boots to a solid grey screen...

[Winston]

Found it:
http://www.apple.com/support/macbook/hd/...extension/

Apple has determined that a very small percentage of hard drives that were used in MacBook systems, sold between approximately May 2006 and December 2007, may fail under certain conditions.

If your MacBook was purchased in the date range listed above and shows a flashing question mark on the screen, please take it to Apple or an Apple Authorized Service Provider and they will examine the hard drive to confirm if it is eligible for a replacement, free of charge, under this program.

Identifying Affected MacBook Systems
Symptom
A flashing question mark appears on screen when the MacBook is turned on.

Affected MacBook models
13-inch black and white MacBook models with the following processor speeds and hard drive capacities:

Processor speed - 1.83GHz, 2GHz, or 2.16GHz
Hard drive capacity - 60GB, 80GB, 100GB, 120GB, or 160GB