Plenty of shows taped in front of a live studio audience still sweeten it with a laugh track. But I have no idea if BBT does.

edit: wikipedia says yes. "Sweetening has been used in a number of television series, from older shows like Happy Days, Taxi, The Mary Tyler Moore Show, to newer sitcoms Two and a Half Men and The Big Bang Theory" http://en.wikipedia.org/wiki/Sweetening

There must be a 'LAUGH' sign - because they are on cue even when the joke isn't remotely funny.