Ask YC: When will FaceTime be open?
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Plus they're selling the new iPhone with FaceTime right now so they wouldn't want to give a company like Google the chance to quickly implement it. WWDC 2011 is also when they'll probably announce the next gen. iPhone which presumably will have new features to focus their marketing on.
This way, other vendor will be seen as 'bandwagoning' Apple (by the general public) as they implemented FT year/year-and-a-half later.
Of course Apple created FT. And of course any other vendor implementing it will be 'bandwagoning' Apple. It's just the public who Apple needs to make clear that they lead, and others follow.
Late 2010: FaceTime support on iPod Touch
Early 2011: Support on new iPad revision
WWDC 2011: 10.7 announced, support in iChat, API's available
Late 2011: Open specification released for 3rd parties.