For a knife or MT based saw, I like a slip stone like this:
http://www.spyderco.com/catalog/details.php?product=91
I do prefer the medium grit which is no longer available, but the fine will do an excellent job, just slower. Natural stones are also available in the same shape. Flat diamond sharpeners like the DMT that dougwalkabout linked above would also work. DMT makes several styles that will fit into small places. Others probably make them too.

With any saw wear will ultimately make it not work well. The loss of set is more of a problem with knife or MT based saws. On some of the Leatherman tools the blades are replaceable at home. On others you would have to send it in.

Respectfully,

Jerry

p.s. The owner of Spyderco is an old friend and I have helped them in their business for years.