Well, I can say now that the problem with my Spyder last month was a broken valve spring.
The piece the valve rides on stayed in place, though, so there was no damage to the cylinder or piston. He said it was probably lucky it happened at idle, and not while I was going down the highway. I was also lucky it happened under warranty (just short of the 2-year mark and before the extended warrenty kicked in). Otherwise it would have been about a $3,000 repair bill.