Now that would be the FB killer moment imho. If you could post to, and read status updates from, and otherwise interact with your FB contacts through G+, without the need whatsoever to log into FB, then FB would be seriously threatened.
I doubt somehow that it would be feasible though.
I've never really looked at it, but Facebook has to have some kind of public API to support mobile apps and so on, so it should be totally possible. Of course if G+ tried to do that I'm sure Facebook would block them or sue them.