I believe the idea is that she will be appearing in the last of the specials, and thus will be the linkage from 10 to 11 and series 5 companion. My info is a bit out of date, though, I haven't been in touch with some people for a bit, so this could all be bollocks.
My complete guesswork prediction is that Donna will return, as will the Master, aided and abetted by someone who we think is dead, but we all know who she is.