Tying the knot, Vegas style, usually connotes images of inebriated couples, Elvis-impersonating pastors, and the kind of bad judgment your mama always warned you about. No wonder Sin City nuptials are such a favorite subject for screwball romantic comedies. The latest, What Happens in Vegas, starring Ashton Kutcher and Cameron Diaz, joins a pantheon of memorableand regrettablewedding flick pictures that have improbable couples uniting in the unholiest of matrimonies.