If there were a simple way to do it with as few parts as possible, I would implement it. But that's a different tangent than a serial data interface...

I think all the wishes expressed in this thread could be met by a cheap PIC microcontroller and a small handful of discrete components. The hardware could be implemented on stripboard with a total cost of about £10.

The downside is that someone has to write the PIC code and others will need to somehow program their own PIC.
On the upside, it would be simple & cheap, the wheel circumference & pulses per revolution (or most likely, some user calculated magic number) could be input over the serial IF using a laptop and stored in the PICs EEPROM thus avoiding the need for custom code or DIP switches etc. Spare pins on the PIC could be pre-programmed to change state at preset speeds e.g. 30mph, 50mph, 70mph and so on.

Is anyone going to design this? Well, I think Octane said above that he might. Failing that I will set about tackling the design in about a month's time (I am currently on about day 7 learning all things PIC). In any event, I suspect most seasoned PICers with an armoury of reuseable code could accomplish this in an afternoon.