Sixofnine raises a very interesting point. The real problems could start when they try to kick somebody out. The conventions in my area are always held in a publicly owned facility. If I just go in and take notes or something, I don't think they can make me leave.
If all you did was sit down in an orderly fashion and take notes, they probably wouldn't try to make you leave. If, on the other hand, you stand up at your seat during the keynote address, strip naked, and start urinating into the crowd...or something even more obnoxious than that, like distributing literature that disagrees with Watchtower teachings...then they'd probably have to give you the old heave-ho.