I don't think it comes as any great surprise that Jason Reid reports, "Livingston is favored to start":  

Barring injury, third-year point guard Shaun Livingston is expected to start ahead of Sam Cassell, who is beginning his 14th season.

Although Cassell was slowed because of a strained left calf in the exhibition season, the move would be based on Livingston's strong play to this point.

Whatever Shaun may have shown us in the preseason, this is more about defense than anything else, isn't it?  Shaun has quietly emerged as the team's second best defender on the perimeter and the wing.  Sam, who will be 38 in November, can't move laterally, can't keep his man in front of him, can't possibly heed every rotation - particularly against pass-happy PHX, and is basically fucked in defensive transition.  Yet, as a clutch, streaky shooter and a wily PG able to find holes in an opponent's second-unit defense, he's a natural sixth man. 

Dunleavy will almost certainly put Sam and Shaun are on the court together at times, with Shaun at the 2.  It worked during the postseason and it's a great way to hide Sam on the defensive end, both against half-court teams like Denver, who tend to play a more compressed brand of half-court ball, and against PHX, who will go even smaller in the second-unit with Barbosa in the backcourt.  The other beneficial thing about having Shaun at the 2 (or the 3, for that matter) with Sam at the point is that it allows Shaun to post up on smaller guards.  Remember how Shaun, after being initially reluctant, went to work against Boykins, Barbosa, and Nash in the post?  By the end of those series, both DEN and PHX were sending double-teams at Shaun; but since Shaun is so long, he was able to find the open guy on the weak side, or -- better yet if they sent help from the block - whichever Big was matched against man coverage. 

I suspect that Dunleavy may use a platoon at the PG --  having Cassell start against slower-tempo teams like Denver and the Lakers, but going with Shaun against more frenetic backcourts like PHX and GS.