I brought this up in Matt Maroon's post about the topic back in January, and he brought up a good point: People like to own their car because it's a moving storage locker.
http://mattmaroon.com/2011/01/03/google-will-become-an-ai-co...
http://mattmaroon.com/2011/01/03/google-will-become-an-ai-co...
But I think the removable trunk will be combined with a passenger-less unit that carries the trunk back to your house and drops it off. Sort of a tiny version of container shipping.