Is General Petraeus going to also be simultaneously appointed as the next Chairman of the Joint Chiefs of Staff? Or will the rank become official upon his retirement?

Because Petraeus will outrank him and/or the Army Chief of Staff otherwise. And all the combatant commanders. Won't that be awkward unless Petraeus is outside the normal chain of command?

Five star ranks were given during WWII partly for protocol reasons as Marshall, Eisenhower, Nimitz and MacArthur had to interact with and command foreign counterparts of higher rank than the prewar Army and American tradition had permitted, excepting General Pershing's special status as General of the Armies ( which he dispalyed as four gold stars) which gave him seniority even above the newly created five star ranks. Or six, had that rank been created, which was discussed but never came to pass.

Another reason was the magnitude of the conflict of WWII where 12 million Americans served in the armed forces. That was war on an epic scale.

Should five star ranks be handed out to dominant commanding generals or admirals in a war who have great successes? Probably, but very sparingly. If Petraeus manages a "win" (i.e. stabilizing ) in Afghanistan, I'd say that he has earned it but some thought needs to be given as to what assignment he will do next once he holds such a rare and exalted rank.