My N10 was locked in Portrait after upgrade to 5.0.2 (Was working before upgrade. Does go into landscape on boot, but never comes out of portrait). Maybe lost it's calibration on upgrade?!
Reboot, or turning off portrait lock and on again, wouldn't fix it. forums.androidcentral.com/google-nexus-10-tablet/339398-google-nexus-10-tablet-auto-rotate-not-working.html says a reset is required.
Ran SensorGuru to check accelerometer/G sensor (looked to be working fine). After turning tablet around quite a few times for a few minutes, it now seems to have FIXED ITSELF!... YMMV!