An article in the August issue of Energy Policy suggests that people idling their cars (leaving the motor running when stopped) accounts for a surprising and mostly unnecessary 1.6% of US greenhouse gas emissions.
Drivers who were surveyed on average thought that a car could be idle for almost 4 minutes before it was better to turn the engine off (in fact, it's more like 10 seconds).
There are lots of resources out there on how to reduce your fuel consumption (in the US they call it 'hypermiling') - and turning your engine off when stopped for a while has to be one of the easiest.
The NRMA reckons that if you're stopped in traffic for any length of time you should turn the key to the 'accessory' position (not completely off) and shift automatic transmission to park or neutral. This way, when the traffic starts moving again, the engine starts straight away. Vehicles fitted with this feature as standard typically reduce fuel consumption in urban areas by up to 15% (see its PDF petrol saving tips).