If the BBC decided to do a spin-off or somehow introduce a new Time Lady I'd be for it, but changing an established character like that for the sake of it would really put me off watching. One of the reasons Doctor Who is popular with so many people is that he's a bloke which helps get a lot of people watching it. Young boys (as well as sme older guys) watch it because he's a male role model they want to be like, the dads watch it for the companion and the mums (and no doubt some of the older female fans) watch it because of the Doctor.
Also, if the Doctor was female, it would screw up some (ok, a lot) of the time travelling. Like it or not because the Doctor is male, he can easily go into the past and take command of a situation where as a woman would more likely be ignored.
Finally if The Time Lords switch genders, what must romances of Gallifrey be like?