This is really huge.
The official announcement was made today (over the night in Singapore). Google have got the support of networks such as Friendster, LinkedIn, Plaxo, Ning, Hi5. And application makers such as Slide, RockYou, iLike and Flixster.
The latest, and biggest, news (a few hours ago): MySpace has joined OpenSocial. I think this is a really huge announcement. MySpace, the biggest network, has teamed up with Google on this. I really hope it will be a success.
What does this mean? Developers only need to create one application, for use everywhere (or maybe a second one for Facebook). End users will now have more applications available. Networks get more exposure.
Opening up the platform can only be good for everyone (other than Facebook, probably).