I like the idea of small compasses that can slide onto a watch strap. I figure that if I do end up having put the compass to use I'd have somewhere to keep it.

That said, a ways back I picked up a Coughlan's wrist compass for a backup (shown in this thread), but unfortunately it didn't last. I had packed it into one of my small kits and when I went through it recently I noticed the compass was no longer working; the disk in it was jammed and no longer rotated. I've seen the Suunto one Mark mentioned and thought it might be a good replacement, but haven't picked anything up yet.

On a related note, I've been practicing using the sun for direction using my watch and that'll have to suffice for my back up for now. Of course, in addition to my real compass I usually have my GPS (no built in compass) & powered-off iPhone too, so going without the button-sized backup maybe isn't that big of a deal .