We've heard the rumors. The rumors are that next week, when Episode 314 "Wishin' and Hopin'" airs on February 1, Ellis Grey will be lucid. TV Guide seems to confirm this in their description of the episode in the Jan 29 through February 4 issue:

As Meredith's mother, Ellis (guest star Kate Burton), who suffers from Alzheimer's, experiences a change in her condition, the Seattle Grace doctors prepare for a change that will affect their own lives: the imminent retirement of the Cheif.

This sounds as if she will be lucid! Very exciting news! I have a feeling we'll either be finding out tonight, in the promo for next week (ABC's promo department seems to be good at ruining things like that), or, we'll just have to wait til next week. So stay tuned!

EDIT: As I predicted, ABC's promo department gave it all away, and, yes, it is confirmed that Ellis Grey will be lucid in 314 "Wishin' and Hopin'."