Just printed out a new Z probe arm that's shorter and sturdier than my old one. http://www.thingiverse.com/thing:201406

I adjusted the parameters so that it places the switch just 1.5mm below the nozzle.

SENDING:M48 n 20 x 99 y 99
M48 Z-Probe Repeatability test. Version 2.00
Full support at: http://3dprintboard.com/forum.php
Mean: 1.497163
Standard Deviation: 0.001727
echo:endstops hit: Z:1.50

SENDING:M48 n20 x99 y99 E
M48 Z-Probe Repeatability test. Version 2.00
Full support at: http://3dprintboard.com/forum.php
Mean: 1.509900
Standard Deviation: 0.003012
echo:endstops hit: Z:1.51

I'm quite happy with these numbers.