A few weekends ago while out on the trails a rock took out my valve stem leaving me with an instant flat tire.
I was able to fit three tire plugs into the valve hole and get enough air back in to get out of the woods.
This mishap shortened my day trip which was a 3 hour drive from home, definitely an unpleasant experience.
I ended up finding some Flush Mount Valve Stems that so far have worked out great.
I hope this info might help someone else out. The 4 pack of valves cost me $8.00 from a local tire store.
This link shows an animated image of the valve system;

http://www.cbperformance.com/catalog.asp?ProductID=1213