The following information was posted on allpar by an insider:

"Neither the 6.4 nor Challenger convertible have been "officially" confirmed. When the 6.4 was given the green light (internally) for a production version, I posted details here per which models it was being assigned to. Last I checked, the 6.4 was still a go, although I have not been able to confirm any firm production dates (yet). I do know that the 6.1L will continue into the 2009 model year (Grand Cherokee, 300C, Charger and Challenger). I still believe that the 6.4 will debut first in the Challenger, possibly in a special model, and most likely the 2010MY. Should know more in the months ahead."