With the first ball of the match a batsman clean bowled. But he correctly protests the umpire didn't call "Play" before the bowler delivered the ball. What should the umpire's decision be?
If the batsman has clearly faced the delivery, I'd say out.
As a proper umpire, not calling Play is a big error, and a contravention of Law 16. I'd be ashamed of myself, but it wouldn't change the fact that the game had started, because both sides considered the ball to be in play (see Law 23, section 1, part b for a description of when the umpires can consider the ball to be dead).
And crucially, Law 23 section 5 states:
The ball ceases to be dead - that is, it comes into play - when the bowler starts his run up or, if he has no run up, his bowling action.
This is the overriding factor - the batsman can't rely on a technicality to reprieve him or her if they've faced the ball. I can guarantee they'd pitch a fit if he or she spanked the ball to the boundary and the ump then declared it dead!
Law 16 is clear. The match / day / session does not commence until the Umpire calls "Play".
Law 23 refers to the ball becoming "dead" with the calls of "over" or "time" but as play hasn't been called in this instance it is not yet "alive"
Law 23 also refers to the ball ceasing to be "dead" with a bowler starting his run up / delivery action if no run up. Again this is interpreted as a ball having already been in play.
Law 22 refers to an over commencing when a bowler starts his run up / delivery action if no run up, but as play hasn't been called this should not be viewed as a legal delivery.
The overriding issue here is the ball not being in play until the call of "play"
This should also be viewed as bad match management of behalf of the two umpires.