I have been intermittently follow this technology for several years mostly because the same thought had occurred to me back in the mid-nineties. As has been mentioned a few times, this is essentially the same technology as is being used in wave generation. Here is a link to a Rockwell abstract from a few years ago.
Frictionless Linear Electrical Generator for Harvesting Motion Energy - Storming Media
The reason I mention them is because they came up with an extremely low friction coating which I can no longer find a link to that I helps to increase efficiency.
Following is a link to Masters Degree thesis from 2004 investigating the same idea.
Title page for ETD etd-11052004-145157
I really like this idea but would like to see actual test data to believe it. From the information released that states a max power output of up to 50KW, one would think a large buffer(maybe supercap) would be needed also. The other thought I always had was that in a particularly windy area, if the vehicle were left outside a lot then it would also be able to partially recharge in the wind.
Or if you were a bit low on charge, get a few friends to help bounce the vehicle up and down for a few minutes
couldn't hoit.
JJ