In reply to iani:
Swamp things are good, but pretty draggy and crap in anything but mud. You may also struggle to get hold of them cause I think they've been discontinued.
Continental baron in black chilli compound I've found to be excellent in mud, and good all round. expensive though.
Bontrager mud x's are ales decent, and cheaper.
Best pure mud tyre I've used is the panaracer trail raker, but it's quite skinny, and not an all rounder.