In 2009 at my Operation Mason Relic event I brought in 6 authentic military vehicles....Humvees, Duece, Unimog, Iltis, etc....the players loved them, and got a lot of use out of them. I had designated drivers, with distance rules and everything worked out great. In May of 2010 at Operation Mason Relic II, I will have 10-11 vehicles that will be used, including one of these.
http://www.military-collections.com/.../HalfTrack.jpg
I think vehicles add a lot to to whole experience, and when used right (safety first) can be an awesome addition to almost any event.