Generally speaking for MTB's you use a short (or medium in Shimano) cage if you have one or two chainrings and a long cage if you have three chainrings.

The best one is the one that suits your set up.

To decide on which cage length is best suited to your needs you need to work out what your total difference of your drivetrain is.  To find this you take the smallest ring from the biggest ring and the smallest sprocket from the largest sprocket then add these together.  So for example -

(44-22)+(32-11) = 22+21 = 43

On an XT mech a long cage has a total capacity of 45 and a medium cage is 33.

You can get away with running a medium cage on a triple set up as long as you avoid the extreme gears ie large ring to large sprocket.  I always run a long on a triple as I don't trust myself.

Not many other people trust you either P-Ness, except when it comes to Shimano advice.

To hijack a thread (but I think Martin has got what he needs to know so I don't feel bad)...

I am gonig to be running a 26-36-Bash set up and was wondering, would shortcage work with them d y'all reckon?

I ask as I've got a nice X9 sitting in the parts box not doing a lot....