Ok so I recall a thread in which tzatziki sauce was suggested as a dipping sauce for spanakopita and a few people were actually "disgusted" saying this was a totally insane combination. I was looking through the Balducci's catering menu for a holiday party and saw:
Spanakopita
Spinach pies served with yogurt-cucumber dipping sauce.
Apparently it's not that insane.
Actually, it is a totally insane combination to an actual Greek. Good spanikopita needs no "dipping sauce". Raw carrots do, though, and tzatziki is good for that, and a great many other things.