That's one of those urban legends i've heard a million times that is in no way true. Every source i've found says that idling for more than 30 seconds causes more pollution than restarting (some say 10 seconds), and the engine wear part is pretty bunk as well IMHO... this is a warm start.

http://www.anr.state.vt.us/reflect/mar1102.htm
http://www.drivecleanacrosstexas.org/help.stm
http://shellne.com/enviro.html#ques8
http://216.239.33.100/search?q=cache:5EwAuZ2ESYIC:prairie.sierraclub.ca/idling/+idling+less+gas+starting+engine&hl=en&ie=UTF-8

there's TONS more....

the car talk boys say more than three minutes, but what the hell do they know. =]