I am unsure of how the surprise round works when only the ambushing party is aware, but will need to actually move to get within melee distance of their ambush victims.
For example: a group of orcs are about to ambush the PCs. They have succeeded on their Hide checks, and will be revealed only when they choose. If the orcs burst out of their hiding place and attack the PCs (assume it requires at least one movement action), does the surprise round begin...
- When they come out of hiding, meaning that they traded their surprise round standard action for a move action that gets them right next to the PCs, and then initiative is rolled without them having actually made an attack?
- Once the move action that uncovered them is done, meaning that they get to swing their swords before the initiative, after having moved?