What effect does changing the stem length have on my bikes handling I'm finding that when jumping my mountain bike seems to be very nose heavy no matter how far back my saddle is. What effect will moving to a shorter stem length have?

My bike, a Boardman FS-PRO came with a 100mm Richey stem. I think that moving to a shorter stem length will allow me, and my short arms, to sit further back on the bike and hopefully make steep dropoffs a bit easier to ride. I've heard that shorter stems make the steering quicker too. Will this make my bike unrideable?
A shorter stem does make your steering quicker, it also enables you to sit more upright and gives more leverage on the handlebar.

The longer stem gets you more forward and in a better position for climbing but I think it makes low speed steering awkward and is uncomfortable for long stretches on the bike.

I went from some ungodly long stem (100+mm) with a flat bar to a 50 mm stem with a small riser bar on my Rockhopper and I'm much happier riding it now.  Pulling up the front wheel is much easier (and of course this helps big time with hopping/jumping).  Also riding off larger drops is easier since you it is easier to hang over the rear of the bike as well.

If you're primarily dirt jumping and free riding, I'd say go with the short stem.   Honestly the only scenario where I would consider going back to the long stem is if I decided to start doing a lot of XC riding involving lots of climbs (NO THANKS!).