HA Holy Grail #1: Occupancy Detection

["Andy Laurence" <andy@xxxxxxxxxxxxxxxxxx>]

I've finally puched down all the CAT5 in Node0, so it's time to think
about lighting.  I've got the switches and lighting controllers sorted, but
what I'd really like is automgic lights.  This leads me in pursuit of the
holy grail.  Occupancy detection.  Has anyone got a reasonably reliable
method of occupancy detection?  I'm thinking a sensitive PIR, which
triggers a timer and turns the light on.  When the timer reaches zero the
light goes off.  Whenever the PIR is triggered, it resets the timer.  I
suspect I can do this with xAP Floorplan, although I've not checked yet. 
Has anyone implemented this?  If so, any recommendations for PIRs?

Cheers,
Andy