Hardly a new rule. It has been agreed by AFALPA more then 4 years ago. It will be effected by 2010, I believe.
In my company it isn`t a rule (yet), but I believe it is a very wise career path to take.
Ofcourse there are companies where this isn`t possible, but if you have the chance to build experience as a captain in short haul, you`d be a fool not to take it.