Garmin offers a speed/cadence sensor for their GPS systems; three little bits of plastic and metal that can tell you how fast you're pedaling and the speed you're going. It can be a bit tricky to mount, though, and without an instruction booklet it can be a bit confusing.
Still, after trying to kludge a few things together, I realized that if I stopped thinking two dimensionally life would be a lot better. That is, if I mounted the sensor thing obliquely rather than (granted, how it seemed to be designed) straight upon the chain stay, I wouldn't be required to jury rig anything.
It wasn't really thinking out of the box. It was more like realizing the box was larger than I thought, and If I looked in a different corner, I'd find the answer to the problem I'd been looking for.
I'm sure there's a metaphor for this somewhere, but right now I have to go do laundry of my dirty running clothes. *sigh* that's one sad thing about winter- both the number of layers and the volume of laundry quadruples.... at least, when it comes to my workout clothes. :P