[ANTSANTS]

As I said in another comment, almost every Megadrive game I've played lets you use both A and C for accept in menus, so you could use whichever orientation you were more comfortable with. I think anyone who started with Nintendo consoles would instinctually rest their thumb between B and C.

My guess is that Sony thought that X and O wouldn't have as obvious connotations outside of Japan, and figured that people would assume the button closest to the player (X) would be the OK button. In practice, I have found that people with very little exposure to Japanese culture still have the same association with X and O in their heads and get confused when using Playstations ("you press X to accept???"), so I'll curse Sony forever for this stupid regional change.

[teamonkey]

This is almost certainly a result of intense focus testing, so I don't think it would be a design choice by Sony based on cultural differences so much as an observation of user comfort and expectations.

That's not to say that those expectations weren't due to cultural differences.

[ZenoArrow]

I always thought the reason for the switch was down to driving games, where X is a more natural fit for 'accelerate'. Also, X to me is a more 'definitive action' symbol compared to O, make of that what you will.