Although the Activity API is included in the documentation (http://developer.myspace.com/community/opensocial/activity.aspx) , it is currently not implemented. Expect that this particular part of the open social API will go through significant changes.

It'll be cool once it's implemented.

J9:

It'll be cool once it's implemented.

Is there any target date by when the Activity API should be implemented?

Not announced yet, but the latest blog mentions a Roadmap with some timeline information coming soon.

The initial release of the Activity API will only support a communication channel between users who both have the application installed.
We don't like to sent out all vampire bite notifications from your friends to the user, if the user has not shown interest in the vampire bite application.
The 'User has installed the XXX application' will be broadcasted to all your friends. (Under the condition that the sender allows that, and that the receiver is subscribed to the sender)

not cool enough