Given how wide open Sarkozy (and for that matter Fabius) has left the center, one would be a fool not to run for it at full speed

Absolutely, and I think this is what she's doing. The two topics she has hit in short succession -- youth and the 35-hour week -- are no mistake, but part of a deliberate strategy. If she moves into the centre, she can still get left-wing support in the second round. And, better still, she can imprison Sarkozy in his own rightist rhetoric. His plan was undoubtedly to fish for a right-wing base now, and move to the centre as the election draws closer. If Royal takes the middle ground, he will have difficulty doing that. At the same time, she's pre-empting on more centrist PS candidates (DSK?) while leaving Fabius between a rock and a hard place, ie between the PS majority on the one hand, and the hard left (who don't want him) on the other.

It's already bearing fruit. (1) Royal is supported by rank-and-file Socialists on her "tough measures for wayward youth" stuff, so it would appear she can bring along support; (2) Sarkozy is making softer burblings (he has just announced that undocumented children will not be expelled from the country, for example) as if he realizes he has to come in from the far right.

I think it's probably a very efficient electoral tactic, and there's no doubt she's consolidating her position as likely candidate. Will the PS split over it? I don't know. How suicidal is the PS left? Where can they go if they break away? My feeling is they'll grit their teeth and stay on.

I don't personally like these "Third Way" tactics, but I admit they may work. Particularly with Sarkozy out on a far-right limb.