I've been eyeing some of these models for some time:
http://www.servocity.com/html/12v_linear_actuators.html
I would love to have a drop-in electronic shifting option for gearboxes, and some of the lighter-duty short-throw actuators there have enough force to shift under load fairly quickly. They are pricey, however, and I haven't had time to find out what type of motor and shaft they are. I called servo city and they were unable to give me any more info than external dimensions, so someone would have to buy one to find out if any of our motors would mount in the same location.