The latest Ifop poll (Le Monde article) seems to suggest Sarko would be the primary beneficiary if Le Pen isn't on the ballot:

Sarko: 28% (-1)
Bayrou: 23% (+4)
Royal: 23% (-2.5)
Le Pen: 13% (+1)
(Villiers: 1%)

Without Le Pen:
Sarko: 34.5%
Bayrou: 25%
Royal: 24%
(Villiers: 3.5%)

Doesn't take a denouncement by Le Pen into account of course. That's the first poll putting Bayrou at parity with Royal.

"The basis of optimism is sheer terror" - Oscar Wilde

Cambadelis, who has always struck me as one of two guys at the PS who doesnt' have his head you know where (LeGall the other; the only two to predict Jospin might finish behind LePen in 02) had an interesting quote in Liberation this week -- that LePen probably already has his 500 signatures but may not run, preferring to campaign informally against "the system" and claim he was undemocratically (sic) exlcluded, rather than finish 4th.

Sarokzy's camp is trying to avoid that outcome at all costs, because he needs LePens' voters in the second round if its Royal -- and moreover because LePen not being on the ballot and blaming the incumbent government for keeping him out could very well put Bayrou over the hump ahead of Royal in the first round -- and likely give him the Presidency.

What remains to be seen is if Sarkozy and the UMP will be held responsible for explicitly calling for mayors to sign for LePen.