Is there a special boot mode to boot into a system with root access (on Galaxy S)? What if the Samsung Galaxy S I've got has already been rooted?..

Is there perhaps a special boot mode which would boot into the system with root access?

Or normally, a "rooted" device always boots the system with complete root access?
There isn't a special boot for rooted devices AFAIK. Any rooted device necessarily boots with complete root access.