Let the games begin. Facebook has had the stage all to themselves for six months with their application platform. That's enough. Enter Google last week with OpenSocial
Exactly who is it open for/to? Developers or users? So far it is an API that a bunch of different players have said that they will adopt in order to support third party application developers. That's a fine start and it reduces friction for the developers by creating better widget portability. But OpenSocial's real openness test is going to be whether I can bring my friends along from one network to another. And bring along my profile too. Or see my newsfeed from one site on another. etc. etc.
The "openness" will be determined by the networks that choose to adopt the OpenSocial APIs. They don't have to support the full API so lets wait and see what they choose to support before anyone declares it open.