This was not a random occurrence. It has happened twice, both times making the disk unrepairable.
Machine has 2 gig RAM, 512 MB dedicated to Windows VM, 250 gig hard disk. There is no other software installed other than:
OS X 10.4.10
Parallels 3
Windows XP SP2, fully patched
Windows Live Onecare (virus checker)
The machine froze while in Mac Safari (spinning wait cursor) which could only be resolved by turning off the machine. When it was turned back on I had the folder with the question mark in it, and Disk Utility could not repair the disk. The first time it happened I could not even select the disk in Disk Utility (disabled).
Like I said, this scenario has happened twice and I will not be reinstalling Parallels until I get some indication that the problem has been resolved. In the meantime I will seriously consider VMWare's product. What are my options for a refund of my upgrade money?