In the tenth episode of the fourth series ("Midnight"), after the shuttle is shaken, we see a very brief cameo of Rose Tyler on a screen (about 16:30 into the episode). Although the feed appears to be mute, you can see her saying "Doctor" twice. The Doctor doesn't notice her, nor does anybody else.
It could be a warning to the dangerous situation ahead but it could also be a warning to the upcoming arc. Is there more information about the meaning of this?