There is an excellent review by David Hawkes in the May 4th issue of the London Times Literary Supplement that asks that question and reviews the history of the notion, particularly as it applies to fiction. (Unfortunately the online TLS site will not let you in to read the whole article unless you subscribe, so I'll summarize a few of the arguments.)

Apparently the idea that words could in themselves constitute treason, even if they did not result in any treasonous acts, was widely accepted in Elizabethan and Jacobean England. In fact, treason was largely defined as verbal rather than actual; to imagine doing a treasonous thing, even in a work of fiction, was tantamount to actually doing it.

There were objectors to this. Our own Medievalist's favorite guy, Sir Philip Sidney, wrote: "For the poet, he nothing affirmeth, and therefore never lieth." John Donne argued that the king had no power over an individual's conscience, and this idea percolated through such works as Milton's Areopagitica until ultimately even James II, no friend of republican notions, decreed that no one would be charged with treason "if hee breake not out in some outward acte." It was the beginning of our modern concept of free speech. Words were free -- it was acts that mattered.

So as I listen to the current crop of pundits on TV argue about what is right and proper to say about Iraq and whatnot, and read how Ann Coulter and friends call various liberal ideas "treasonous," I think it's worth asking: can mere words become so powerful that they constitute treason?