The life seems glamorous, the engagements pile up because of your fantastic book or your wonderful experience, but what is the real story?
Answer? Don't give up your day job! There's excellent coverage of the economics of public speaking in a recent post on Ben Casanocha's blog: So you want to get on the speaking circuit.
There is another plus of public speaking that Casanocha doesn't mention - public speaking can attract people to whatever you are doing in that day job - the courses you run, books you have written, great causes you espouse and so on.
By the way, my link to this post came from Tyler Cowan's blog: The Marginal Revolution. It has been described as the 'best economics blog in the universe'!