I ride with a 100mm stem and wanted to see if a short one made any diff. So, just last week, I borrowed a 60mm one from a friend and it made a noticeable diff. to the handling, more snappy and responsive, but only slightly. There was no noticeable negative effect as I angled my bars slightly more forward to give roughly the same cockpit room.
I have the standard diameter alloy Easton 70 monkeylite hi-rise bars, so I'm looking at a Easton vice stem @65mm (they do an 80mm as well). Stems are easy to swap, so I'm thinking 100mm for long XC days and the short 65mm for DH, trail centres and playing in the woods. Chances are, I'll experiment with both and eventually stick with one or the other.