"This expression comes out of the US Air Force test pilot program of the late 1940's. The envelope refers to a plane's performance capabilities. The limits of the planes ability to fly at speeds and altitudes and under certain stresses define what is known as its performance envelope. It's an 'envelope' in the sense that it contains the ranges of the plane's abilities... Because 'pushing the envelope' has such an esoteric origin, it took a best-selling book -- Tom Wolfe's 'The Right Stuff' in 1979 -- and later the popular movie 'Top Gun' to introduce it to the general public. Since then it has begun to crop up in increasingly non-technical contexts, to the point where it is now a currently trendy metaphor for simply 'pushing it,' or testing the limits of what is permissible in a given situation."

Tho the source doesn't mention it, the 1940's test pilot programs were largely focused on breaking the sound barrier. Thus the envelope in question was likely the dividing line between sub-sonic and super-sonic flight.

Lord knows somebody's gotta' "push the envelope." If not Conan, who? The nightly news has demonstrated they can't.