Get a cheapo discontinued wireless one (think Nashbar/Performance has the old Cateye Cordless for $35 or so), mount the sensor on your fork, then put the computer in your pocket...inconvenient for current speed, I suppose, but you can pull it out of your pocket at the end of the ride so you'll know how far you went.

At the risk of sounding like a total loser, then again perhaps I am, I'd love to have cadence available when I'm spinning down a hill. It's easy enough to look at a chart when I get home, but I'd like to know it in real-time without having to do math...."gee, if I spin just a bit faster I'll be at 150 rpm"...something along those lines.