I've actually been involved with the OpenSSO project during the last 2 years or so, and I honestly don't think it will disappear at all. It had a very active and vibrant community which supported it, many of which have already made the jump to help ForgeRock.
On top of that, OpenSSO/OpenAM already has some terrific features. Its Agent interface is superb, the SAML engine is rock solid, FEDLETs are ahead of their time, and it even had a well documented API for integrating directly into your own application. That's not to say that OpenSSO didn't have room to expand (I found its STS service to be "finicky"), but I expect many of these issues will be addressed by ForgeRock.
I understand that Oracle already has it's own IAM suite, but I think dropping OpenSSO will be something that they regret.
OpenSSO is dead. Long live OpenAM!