The May issue of Empire Magazine has a quote from Rick McCallum on the digital version of 'Revenge of the Sith' (Pg 78):
"The digital version of the movie will be different from the version that goes out on film. That has to be locked down so much earlier, whereas the digital can go right up to a week-and-a-half before the film comes out"
The digital version of 'Attack of the Clones' was also different to the version that came out on film, the most notable difference being Padme holding Anakin's mechanical hand during the wedding scene.
But will this digital version of 'Revenge of the Sith' be longer than the version on film, perhaps as a reward to theaters that have upgraded to digital projecters, and fans who choose to see the movie in a digital theater. Time will tell. Thanks to Rich for the alert.