Here is a video we made a couple years back. It was performed exactly how they do it in the industry. I think JR even supplied the banner, which is a scale "billboard" type aerial banner. My friend who flew the banner is a JR rep. Call them and ask them if they have them for sale or however they distribute them. I can find out where he got the banner if you want, they are made a certain way to fly aerodynamically. Don't try to do this with just any old sheet, they have to be made a certain way. Google for more info or if you want email me and I can provide you with information on aerial banner flying.
I have some more videos of it if you want to see it. The plane is equipped with a quick disconnect, again, just like real aerial adveristers have just in case there is a problem with the plane.
PS - No weapons were used in the making of this film.
http://www.youtube.com/watch?v=6EUBaF-EStw