Many conferences have wifi for the audience these days. People blog the conferences or chat during the conferences. There is definitely a back channel and a lot of people who track conferences online. At a recent conference in Helsinki, Kevin Marks, who was in California, wrote a limerick heckling Tom Coates on IRC. The difficulty is feeding some of the good stuff back to the speakers. This is where HeckleBot comes in. HeckleBot is an IRC bot that sits in the IRC channel for a conference. You give it commands like "?heckle Stop pointificating!" on IRC. The bot talks to a linux box connected to an LED display facing the speakers. The LED displays the message to the speakers. This way, the speakers can get immediate feedback from the audience as well people watching a video stream or reading people blogging the event.
I promise to try to get the HeckleBot set up at as many conferences I attend if people will help me build it. There are some links to the various pieces on the wiki page about HeckleBot. Please sign up or contribute on the Wiki.