Again, let me preface this by saying I would have no problem with a Female Doctor.
I do think it would be unlikely. Romana, and others, it seems, have the capability to choose their new face. Offhand, River is the only other Time Lord I recall offhand who chooses what to do with her Regeneration, so, maybe it's a female Time Lord Trait, to have control, rather than a Time Lord Trait in general?
But, anyways, the Doctor always regenerates in a crisis, and seemingly has no control over it, he just gets what he gets. I would think, under those circumstances, the Regeneration choice is feeding off who he is deep down, and I think he clearly identifies as male Humanoid, since he always regenerates (so far) into a male Humanoid. After 10 (or apparently 11 Regenerations), he hasn't changed into a female Humanoid or a slug or a plant creature, so, I don't accept the "Statistical probability dictates he'll be female at some point". I think it would be far more likely if he had control to choose to appear female.
But, again, I'm not opposed to it, and wouldn't care how they hand waved it away if they did go female. I would actually very much like to see a female Doctor at some point, as I think it could opn up some great story ideas (Just please no non-stop belching and barfing lush)