When is your estimated delivery code? Have you got any email saying that your order has been shipped? If yes, I say don't lose hope just yet. The day is just starting and hopefully you'll get your code soon, since you're supposed to get one once your order has been shipped (got shipping info earlier this morning, still waiting on code)
On the bright side, there's at least one report saying the user has already got the activation code, so there's definitely hope!
But the Stadia servers went live over an hour ago...so...where's my code? I understand they want to stagger roll-out to lessen the stress on the servers and fix things as soon as they come up...but this is Google we're talking about. There's no reason they shouldn't have already sent codes out for pre-orders from at least day 1 (like me), if not day 2 as well! I pre-ordered super early for a reason...so that I could play as soon as the service launched!