Ben Weiss wrote his master's thesis on adding xy closed loop positional feedback to a hobby printer, check it out:by 100ideas - Mechanics
Ben Weiss' masters Mech E thesis was on "Closed-Loop Control of a 3D Printer Gantry" (pdf). He added linear encoders and independent controllers (based on teensy) to the x- and y-axes of a makerbot-style printer (MBot CUBE). His thesis contains detailed information about the background and implementation of his mod, including circuit and mechanical diagrams, and he has posted source code for hisby 100ideas - Mechanics