If you want to get really outrageous, you could buy the $12 computer off Amazon and write an app to use the N900 camera to "screen scrape" the speed reading off the cheap computer. Seems about as reasonable as trying to hook it up to a magnetic sensor in any other way.

I plan on using the N900 as a bike computer, but hadn't even considered trying to hook it into a magnetic sensor because it's so far out there.