"Jeep is coming out with an all-new 2nd generation 5.7L MDS Hemi for the 2009 model year. The new Hemi will feature Variable Valve Timing technology and other new advancements, resulting in notable gains in fuel efficiency, refinement, power and torque. Specifications will not be announced until sometime next year. The new 5.7 will be Standard on the 2009 Grand Cherokee and Commander Overland models, and will be optional on the Limited models.
The second all-new Hemi engine will be a 6.4L powerhouse that will supposedly put out more than 500 hp. While not officially announced, the new engine is slated for 2009-2010 SRT8 models of the Grand Cherokee, Chrysler 300C, Dodge Charger, the new Dodge Challenger, and one or two other Chrysler/Dodge vehicles."
It looks like anyone who wants a 6.4 will have to wait for the 2009 model.
See:
http://www.wkjeeps.com/wk_updates.htm