There's a thread at Apple's Discussion Boards about an issue that's bothering a number of users. Specifically, the current model 17" MacBook Pros seem to be experiencing hard drive lag. Several users report hearing the hard drive park itself, but at inappropriate times, causing the Marble of Doom to appear and bringing all tasks to a halt. This comes a short time after other users have reported a strange beep.
One user reported no red flags while running Activity Monitor while another went so far as to swap the hard drive to no avail. If you're having this issue, or if you've solved it, please let these folks know. Update: A user calling himself theycallmefreddy reportedly fixed the problem late this morning by creating a 2nd user account. You can read his post here.