I like the idea of the user name on the car for meets and shows but im not sure about on the road. Mainly due to this tragic incident involving a Civic Type R meet:
http://www.pistonheads.co.uk/gassing/topic.asp?h=0&t=377548&hw=10penceshort
The incident itself has been discussed to death on here so I would rather it wasn't rediscussed now, but what I would say is if you were involved in some kind of accident it wouldn't be hard for the Police to see you what your attitudes etc are towards speed, and in the link above it illustrates how posts made on an internet forum were used as evidence in court.
The other night when you posted on here about how you were having a play with another car on a DC and he spun out it wouldn't have been hard for the other driver to find out abit more about you with the web address and user name on the back of your car.
Im not slating you or your driving etc and I like the idea, Im just outlining the reasons why I chose not to do it to mine.