Pointsec have been doing a survey for 5 years now which calculates the number of lost mobile devices in Taxi/Cabs in US cities.
The survey found that during the past six months, nearly 12,000 electronic devices were left in taxi cabs in the San Francisco-Oakland Bay (3,106 devices) and Washington, D.C.-Baltimore (8,701 devices) areas.
The most frequently lost items were in order of size - Mobile Phone, PDA then laptop.
What did suprise me is the honesty of lots of people as in Washington DC - 80% of mobile phones eventually found their way back to their rightful owner!
You can read more details about the survey HERE