Nick Diaz will vacate his Strikeforce title ahead of his bout with Georges St-Pierre at the end of 2011, preventing a title unification bout and likely closing the door on his return to Strikeforce. A tournament could ensue in Strikeforce for the WW strap or a top contender bout (a-la Jacare after Shields vacated) could take place.
I would wager that Strikeforce would rather keep their title in-house than allow it to be transferred to the UFC, especially to a fighter like St-Pierre who would never fight on a Strikeforce card. So this arrangement was probably a stipulation put in by Showtime/Strikeforce not something Caesar Gracie/Nick Diaz orchestrated.