You can do it with hardware pwm control if you are using a brushless ESC to control the motor. The pwm control can be driven by wiringpi for python using port 18.
If you want to see an example of how that is done, you can look in the Python Gertboard program motor-wp.py which you can get from here...
http://RasPi.TV/downloads
Note that you will have to look up the pwm settings for this. They will not be the same as in the example, which is for a brushed motor on the Gertboard.