You have to win the Royal Rumble. Just be sure you have no Championships leading up to the PPV...
Your thinking of Smackdown! Vs. RAW 2007.
On Smackdown! Vs. RAW 2006, all you have to do is complete the RAW Storyline.
[Edited by king-of-games, 10/13/2007 8:43:12 PM]