The simple answer to that is, nobody can be certain. Although we have been teased that she may be the Doctors wife/lover/murderer etc. There is no way we can actually be sure. She is supposed to appear in the Series 5 finale episodes. In the third trailer released, her, the Doctor and Amy were seen running through Stonehenge.
River said that her and the Doctor were traveling in different directions meaning that everytime River sees the Doctor he knows her less but she knows him more.
And in series 6/02 River makes out with the Doctor.