[nathanww]

There's a copuple of ways you could go about this:
1.Open loop--you calculate how much the robot will move to do whatever you want it to do, and then program the speeds, headings, and distances making sure that you never cross the line.
2.Semi-open loop--you use devices like encoders/gyros to track your robot's position relative to the line and modify behavior accordingly
3.Fully closed loop--you have a sensor that tracks the position of the line. This is not necessarily as daunting as it sounds--keep in mind that although the gaffer's tape is "non-reflective", it reflects significantly different frequencies than the carpet.