The decision to implement the conversation scope within JSF was deferred as it was decided that it would be more appropriate to implement the scope at a level in which it could be shared/accessed across the
Java EE platform. Scopes which provide this level of visibility are defined in JSR-299: CDI. Therefore, the official conversation scope for JSF is indeed the conversation scope defined by JSR-299.
However, because of the order in which the specifications were finalized, JSF 2 wrapping up nearly 6 months before CDI, it was not possible for JSF 2 to provide UI components that would aid in managing the boundaries of the conversation scope like Seam provides. Now that CDI is officially part of the Java EE platform, you can expect that, moving forward, we can take steps in JSF to help support the conversation scope.
One consequence of the decision to move the conversation scope to CDI meant that it's necessary to use JSF 2 in conjunction with CDI. This turns out to be a good thing because the services that CDI provide (dependency injection, managed bean discovery/definition, events, etc) go way beyond the JSF managed bean facility and is well worth the commitment. I recommend this strategy strongly over trying to implement a conversation scope using the JSF 2 custom scopes.