cracked exhasust valve

2001, with a 2.4L engine started running rough today. Did some troubleshooting and narrowed it down to #1 cylinder. Compression is poor, so we looked through the spark plug hole with an industrial borescope, and one of the exhaust valves has a piece missing from it. Approximately 1/8 of the valve is gone, and the piece is most likely downstream in the exhaust somewhere. The car has ~47,000 miles, and is out of factory and dealer warranty. We'll be tearing it down to do a valve job at a minimum.

My main question pertains to the nature of this failure. Has anyone else ever heard of an exhaust valve cracking? Between our group of friends and coworkers we have one very seasoned auto mechanic, and plenty of shade tree experience as well, and none of us has heard of this . Question #2, is there any likelihood of Pontiac covering any cost associated with this? In other words, could there be a known bad batch of valves out there, and Pontiac, or the engine manufacturer has a recall on them? Any help is appreciated.

It is doubtful that Pontiac will cover the repair. However, I would notify the NTSB about the problem as this could lead to a recall of the engine components at which time the repair becomes covered. As far as an exhaust valve cracking it just doesn't happen. Probably a manufacturing defect. Usually they will burn or bend if they hit a piston but not fracture like you said.
If I were you I would find a reman head or get one from a wrecking yard. If one valve fractured you may have gotten a bad set and may run the risk of having it happen again. It may also be much less expensive since the valve seat may need replaced as well as the valve.