Now that i have a little more time heres what i'm thinking. First you should probably familiarize yourself with op-amps. I found this on google:
http://www.colomar.com/Shavano/intro_opamp.html
Once you have read that here is what im thinking. First you can filter the input using a simple RC filter. From there the signal is fed into 2 op amps, the inverting terminal of one and the non-inverting terminal of the other. The other terminal on the two amplifiers is tied to ground( or a potentiometer set up as a voltage divider if you want ot be able to mess with the maximum/minimum brightness). The voltage output of one of the op-amps will vary proportionally with RPMs, while the other will vary inversely. I left many components out in this description, but i will post a better explanation along with a schematic as soon as possible (prolly this weekend.) I really need to know what the voltages read.