Well it was bound to happen.
3 years ago, Google CEO Eric Schmidt accepted a seat on the Apple's Board of Directors in what was thought by many to be an alliance against Microsoft. As the years passed, Google became a powerhouse unto itself with their heavy presence on web-based interfaces and the introduction of the Android mobile operating system, finally accumulating in their announcement of the computer OS; Chrome. They represented a whole new model of computing that neither Apple nor Microsoft subscribed to yet was gaining popularity from the masses.
I suppose it got pretty awkward for Eric at Apple as Google started to pose a real threat to them. Apple, being the closed and highly protective company that they are then began to act out against Google with inane requests such as requiring that Latitude be made a web app instead of a native one.
The situation finally exploded when Apple's App Store rejected the official Google Voice app and pulled all apps that had the Google Voice function, resulting in enough complaints by iPhone/iPod Touch users and developers (and probably Google) for the US government to step in and questions all parties concerned.
So it makes sense that Eric would step down from the board to prevent any further conflict of interests. Apple's statement on Eric stepping down does not directly address the Google Voice debacle but it talks about how it would be impossible for him to remain with the company.
We've always known this would happen and it makes sense that it does now. The battle lines have been drawn and it's Microsoft vs. Apple vs. Google