Instead of the retractable Z probe, now using Force Sensing Resistors (FSR) sandwiched between the glass print surface and the triangle frame, wired up to the unused heated bed thermistor input.
When the hotend pushes down on the print surface, their resistance changes quickly from 500 kΩ to 50 Ω, depending on the force. The raw ADC value on the Arduino changes from 1023 to under 200, and the bed temperature display in Repetier-Host goes up from 0.0 to 240 Celsius.
This allows calibrating the print surface with 0.03 mm repeatability, which is good enough for printing PLA on cold naked glass.
FSR product details: http://www.phidgets.com/products.php?product_id=3104
Source code: https://github.com/jcrocholl/Marlin/tree/fsr
Project history: http://deltabot.tumblr.com