I think a lot of us are often confused about the legal ramification of having a trademark (i.e a Coke bottle) visible on screen. A great article just came out in Microfilmmaker magazine today that explains it in very clear terms, check it out
http://www.microfilmmaker.com/tipstrick/Issue26/LegOnSet.html
http://www.microfilmmaker.com/tipstrick/Issue26/LegOnSet.html