No need to spend big $.
You can buy, for under $100, a nice cromoly Sun unicycle, for pretty much any wheel size you can think of, and then you can buy a knobby tire for it. Total investment under $150, even if you get some nice pedals. Trust me, it'll take you a while to get good enough to do any damage to it. When/if you do, you can get a KH frame and pass the Sun on to someone else.
I don't see unis on the trails here much, but there are plenty of 29" ones, and have been for a while. I built a coker (36" wheel) uni frame for someone once, and he's actually taken it offroad a few times. Said he'd need 200+mm cranks to get up any hills, but it's fun on the downhills.
Hmmm Surly is doing one too. ::Waits for 06 QBP and ponders the thought of riding on one wheel:: That would be very intresting. Any idea on what they are doing for a price?