And a plug for my, now hopelessly out of date, site:
http://www.strangely.org/owned/bikecomp/index.html
I've now got the speedo mounted on the dash, in front of the standard one, with a little bracket holding a bulb over the bike computer. All custom bracketry was courtesy of SpeedySteve.
The speedo has now done about 46,000 miles without fault, and is easily reconfigured for different wheel / tyre sizes. Definitely recommended.