Last week we reported on a thread at Apple's Discussion Boards about an issue affecting some 15" MacBook Pros. Specifically, their hard drives seem to be spinning down and even parking themselves at inappropriate times, like in the middle of a task. It's understandably frustrating, as that would slow things down considerably and offer unwanted "quality time" with the Marble of Doom. According to CNET, Apple is aware of the issue and working on a fix right now.
There's no word on when it will become available, so affected owners should just hold tight for a few more days. Note that one user on the discussion boards claimed to have fixed the problem by creating a new admin account on his machine. We haven't experienced the problem or replicated this fix, so take it for what it's worth.
CNET notes that the issue seems to be limited to the 15" model with a 5400 RPM drive, but at least one user in the discussion board claims to have swapped drives to no avail.