Mr. Awe made an interesting suggestion that "Donna will jump within her own history and save events. The result will be that she loses her mind like Caan or, if she's lucky, just her memory.". This is similar to my (admittedly silly) theory:
The Doctor figures out some way of turning the Time War in the favour of the Time Lords (maybe some info or equipment or something). Donna volunteers to get sent back to the Time War (over the Doctor's objections, no doubt). Due to the fact that "something is binding" the Doctor and Donna together, when she goes back in time, she ends up in the Doctor's TARDIS during the war, to give him the information/equipment (maybe the 'him" in question will be an earlier incarnation of the Doctor (Christopher Eccleston, maybe, or perhaps even Paul McGann?), thus feeding into the whole "three-fold man" thing) The result is that the Time War is changed and the Time Lords win. Thus they exist in the present time and can fix the whole mess with the Daleks.
Anyway, that's enough wild speculation for me for one day.