No jeans at Per Se.
I realize designer jeans cost as much as a mid-range suit, I own several pairs myself, but the point is class and respect. Not to sound like a jerk - but you're eating at the best restaurant in America, if not a matter of respect for you, perhaps some respect for the other diners and chef Keller and his team?
Depends what you mean by "jeans". I always wear black jeans there (zegna, armani). Blue denim, that's tougher, though a denim "suit" would fly. I recommend it, now that I think of it.
About respect, etc. well I respect other people, and I appreciate when people return the courtesy. I don't think per se is even the best restaurant in Columbus Circle, but I do respect them. I don't really respect what they wear, but I respect them.
If you want to appear classy to people who actually come from class, I wouldn't recommend wearing business attire to dinner, but I'll respect you anyway, and I hope they do too.
Oh, and it's fine to get a survey of dress code tempered by experience at chow.com, but I would not recommend getting fashion advice here. Lightning generally doesn't strike twice in the same place.