I guess anything is possible, but thus far Jim has consistently used the formula of: ascension comes with commensurate amounts of power and constraint. As a force of nature, I don' think Mab can lie. As well the world dissemble for no other reason than atoms decided to stop attracting each other based on charge. Human, endowed with free will, seem to be the only ones with the dubious privilege of reconstructing reality to fit their own subjective whim.
The little sequence with the Erlking, and the fact that he considered the option of just ignoring "courtesy" and killing Harry, suggests that courtesy and keeping their word is something the Fae have power not to do. The raw fury Mab displays at Harry's suggestion that she might break her word likewise suggests that for her, keeping her word is a point of personal pride and self-love, not something she is merely forced to do as a Sidhe. This, BTW, is one of the reasons I can't take the Molly-as-Mab theory seriously. A rigid sense of personal honor is something you're pretty much born with or don't have at all, in my experience. It's not something Molly would acquire over time.
It is also one of the things I really like about Mab, so maybe I'm a bit biased. Mab is up there in my book with Talisman/Matthew Maule (Saberhagen's version of Dracula) in personal coolness, not to mention scariness. In Butcher terminology, she is
gadara.
-Max
