I've decided to accept an offer of another job from a different software company, and have an offer in writing.
Tomorrow, I hand in my resignation but also scheduled for tomorrow is a product planning meeting to discuss how we will progress with our program.
Resign in the morning, and allow the CEO to know about it before sprint planning the next sprint, and product speccing the next release, but endure an awkward environment because of this.
Resign at the end of the day, avoiding creating an awkward environment, but potentially having to take part in planning of things I can't possibly ever be part of.
I'm leaning towards option 1, but I'd be interested to see if anyone comes up with perspectives I haven't considered.