Some motor controllers measure back-EMF produced by the motor to measure speed and use it in a velocity feedback loop (so the position doesnt have to derive velocity and the system can be more stable.). Usually position is still measured, so there are two loops. Measuring BackEMF is complicated, it's like you trying to measure a spinning wheel while someone is vehomently trying to spin it. There is filtering that needs to take place, preferrably done in the analog domain. Which means you better know your differential equations.
In my opinion, I think you will still have your work cut out for you measuring the rotation with what Soeren said. If you are just trying to save money, buying the encoder is the cheaper route. You'll throw a lot of $$$ in development for non-encoder measurement.
The current is fairly easy, but still requires a filter, albiet a simple one. Otherwise, you could same during a dip in the current.
Take a look at
http://www.openservo.com , they measure current, backEMF now too I think, and the schematics have some of the filters needed. You might even want to start from that platform and get where you want it. And they would be interested in any contributions you could make.
Colin
