How does E-Test at a PCB Fab work?
As I understand it, they extract a pad-list and netlist from the gerbers, and then have a machine with robotic arms carrying test-probes (Termed "Flying Probes") to manually ohm-out all the connected nets.

See:
http://en.wikipedia.org/wiki/Flying_probe
http://www.spea.com/ElectronicsIndustryTestAutomation/FlyingProbeTesters/tabid/108/language/en-US/Default.aspx
http://www.youtube.com/user/speamovies (Really cool videos)