Everybody's entitled to speculate on tonight's Doctor Who. Why not me?
1) David Tennant will continue playing the Doctor - as the Time Lord will regenerate into the 11th Doctor tonight but keep his existing appearance.
2) Donna the Temp is actually Romana, the Doctor's Time Lord companion from a long time ago. She will regenerate tonight as well ...
Note that Richard Dawkins had a cameo last week , he is the real-life husband of Lalla Ward (pictured below), who played Romana. If so, perhaps Lalla will return to the series to play Romana again in her new regenerated form. In that case , both Tennant and Ward will be playing a new version of the character they have played before. (Does that make sense?)
3. The First Doctor will make an appearance - after all he is the only previous version who has been to the Medusa Cascade.
If I'm wrong on this, and Tennant is leading the series, (shame on him) then maybe the 11th Doctor will be played by Russell T Davis himself... Clues? I'ts obvious that he loves the character, he's just given up the producing role, and he's the best person who could hang around the set and not arouse suspicion :)