To add another data point, I helped teach an undergraduate engineering course where the students built Lego Segways for their final project. Here is a image of two students driving their segways over an obstacle course using bluetooth control:
An important part of the controller is the loop time/update rate. For our Lego Segways, an update rate of 10 ms or better gave good balancing results (this depends on the dynamics of your system). In each control loop iteration, the students had to do the following:
Read sensor information from the gyro sensor and wheel encoder
Numerically integrate the gyro sensor reading to get the segway lean angle
Differentiate the wheel encoder readings to get the wheel velocity
Perform the controller math based on sensor information
Output next command to the motors
While the PI certainly has more horse power than the Lego NXT brick, I'm not sure how much tweaking to the Linux kernel would be needed to meet these requirements! Also, inconsistent loop times will certainly hurt your performance or prevent the segway from balancing. In any case, I'm also excited about the possibilities!