Well, technically,(and I wasn't watching with any interest) didn't Donna 'become' the Doctor in the last episode? And how does the sainted RTD expect parents to explain the subsequent brain-rape to their 8 year olds?

There are good reasons not to have the Doctor regenerate into a female body (not least because it would be trite pandering to the PC element on the part of the BBC - oh...) but if he can do that then why not into any alien body? Hell, why not a Dalek? (I may regret saying that.)