I don't know what Lucas has stated but using a bit of common sense I'd be pretty willing to bet the person Leia is referring to is Padme. I mean, if not - what the hell is that whole scene about in RotJ. Luke wants to know what his real mother was like and there's Leia just waxing lyrical about some woman that has no real connection in the saga, her adopted mother who we see for like two seconds at the end of Sith. That kind of reading on the scene totally destroys any impact in their conversation and really if that was the true intention there would be no point having it in the film. Why discuss someone who in the end is of no consequence to the greater picture. Pointless!!! As stated above by several people now, if you read between the lines here Lucas is showing us that Leia is indeed strong with the force like Luke. She can remember images and feelings of her mother before she died, while Luke remembers her final message - subconciously and inexplicably! It reveals a strong connection between Padme and her children even in death. And yes that's right death - she is dead, ceased to be - she is an ex-Padme!!