By the account of things I've been reading on the CNC Zone, there is a stepper driver product being developed by Mariss Freimanis of Gecko Drives that will allow a step motor to perform like a servo motor, but retain the advantages of a step motor. That's right, a step motor that doesn't lose torque at higher speeds, and yet till has lots of it at low speeds. His early tests show performance to be very close and even better than a comparably sized servo. The product is estimated to be at least six months away, due to them applying for a patent for it.
You can read about it in the following threads:
Stepper Servo
Gecko PID step motor servo