One theory regarding River and 10 is that he had another adventure with her during the farewell tour between "The Waters of Mars" and "The End of Time" - later in his timeline but earlier in hers. Then she would recognize his specific regeneration but not realize that he wouldn't know her.
Another possibility is that this might be the very adventure we see in the 50th, from 10's POV at least. There's plenty of time between "Mars" and "Time" for more 10 adventures. But then it would be unlikely to include Rose, as we know it does, so probably not.