Occupant Classification System
Older vehicles with bo back seat into which to buckle your kids (read pick up trucks) had a key switch to disable the passenger air bags in North America.
The current trend in occupant seating in North America is to move to an automatic system which classifies the occupant and automatically manages the passenger airbag system. This Occupant Classification System will become standard in the US soon. The technology is fairly recent.
I am aware of systems that include 4 weight sensors (in corners of the seak hardware module), a seat belt tension sensor, and a control module in order to classify the occupant.
In europe and other parts of the world, apparently the keyswitch disabled systems are permitted, but not here.
2002 Volvo XC70 Ocean Race Edition, #1/100 (past ride)
'05 V70AWD, Sport package, Titanium Grey/Graphite (Retiring at 300,000 km)
2004 XC70, Nautic Blue (picking it up tomorow!)