Thanks for all the responses.
I can readily believe that the interrupt/surprise situations don't come up all that often.
Let me run one by you though and ask how you'd handle it. Say there's a trap in a corridor, where the first PC to move through an area triggers it, and it changes the terrain somehow. Maybe it releases a poison gas cloud that fills an area of the corridor. Several PC's make posts which involve moving their characters through this area. (This could happen even if you were enforcing initiative order, since several PCs can act and post in sequence in that case too.)
The first one through triggers the trap, and if the others take their posted mves they'll be going through the gas. Do you just rewind and allow them to redo their moves?
Another thing about rewinding like this in general. How do you typically handle rolls for moves that get retconned? Do you keep the rolls as long as the target and powers and stuff stay the same? I worry about things like, the original posted roll would have hit, but if you retcon the move for some reason that causes you to lose CA from flanking, that roll does not hit. Does that affect the player's decision whether or not to retcon? Or, say, you rolled a critical hit. Does that make you want to suck it up and move through the gas anyway, but if the outcome of the attack was still unknown, you might act differently?
For these reasons I'd almost think you should just always reroll if the DM announces the option to rewind, whether the player takes it or not... though that is still less than ideal. It will really stink from the player's POV if he's forced to reroll that critical hit even though he didn't want to change his action.