Réponse à la question en suspens:
par rebond sur un site anglais depuis CI:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00CZZW
"There is also the question to why the D70 doesn't want to sync faster than 1/500s with dedicated flash units. I can see two explanations, the first is the marketing one, it would be too "comepetent" and steal interest from the higher end cameras not having this "feature".
The technical explanation would be that some flash pulses might be quite long (full effect on the SB-800 has a duration of 1/1050s) which could give unexpected results if you expose without having the full flash pulse to light the subject (say when shooting at 1/2000s and the flash is set to full output). The camera has no possibility to know how long each flash pulse might be from all kinds of dedicated flashes that are manufactured."
Au passage, on s'aperçoit que mème au maximum de puissance et avec une obturation au 1/60e, la durée du flash est de 1/1000e. Pas besoin de synchroniser plus haut pour figer!
Et des éclairs moins puissants seront plus courts en cas de besoin!
Cordialement