I think a lot of the commenters here are overestimating the probability of car theft.
"The National Insurance Crime Bureau (NICB) reports that 2009 marked the nation's sixth consecutive year of declining vehicle thefts in the United States. U.S. motor vehicle thefts declined 17.1 percent from 2008 to 2009—the largest annual decline in decades."
Most of this decline will be related to better in-vehicle security systems rather than a more obedient public.
The interesting stats are the thefts of older vehicles - which are way higher than new vehicles because of the crude anti-theft measures in 80's and 90's cars.
"The National Insurance Crime Bureau (NICB) reports that 2009 marked the nation's sixth consecutive year of declining vehicle thefts in the United States. U.S. motor vehicle thefts declined 17.1 percent from 2008 to 2009—the largest annual decline in decades."
http://www.rmiia.org/auto/auto_theft/statistics.asp
Also "premium" cars from zipcar have pretty discreet branding - just some on the rear bumper, nothing on the doors.