I shoot car shows all the time. I have not had anyone sign a release yet. I DO tell the owners that I am shooting a car show video and they are cool with it. I have had people run up to me before and tell me "make sure you get the red Honda in the corner" type thing too... Some people on here will say get a release but I have been shooting car shows for the last almost 7 years and I have not ran into any problems yet. 95% of those shows were shot on Betacam too. So it was very obvious that I was shooting a video.
Yeah... conventions and such generally aren't as big of a deal. I wouldn't worry as much about asking the people who are showing their cars in the show if you can shoot them so much as asking the people running the convention if you can shoot.
The people bringing their cars there are bringing them there with the intent to show them off to everybody they can. This is what a convention is for. The only people you might have a problem with are the people running the convention as they might not want someone shooting or have limited media or exclusivity agreements with other people. Run it by someone involved in organizing the convention just to be safe...