Here's the thing:

For next year's blu-ray, since the format is 1080p Lucas doesn't really have much of a reason to pay for a re-do of the '04 masters at a higher res. Those masters are already 1080. Episodes II and III have been ready to go since the day they came out.

Now, keep this in mind:

Episode I is hitting 3D in theaters in 2012 and ILM is already at work on the conversion process, so they probably did a new DI (or, as we've been debating, went back to the files if they're still there) in order to yield 1) a new transfer for next year's blu-ray (which the film actually needed, unlike the other five) and 2) a new, higher res master from which they could start the 3D conversion process. II and III are already completely digital. When/if they get around to IV in 2015, doing a new DI probably won't seem like a such a horrible financial burden for Lucas /sarcasm.

They really are gonna need to do a fresh scan for the 3D version of the OT. In the case of movies like II and III and Avatar, the cameras for the live-action photography were only 1080p, yes, but they went through 2k DI's as far as I know, effectively scaling the live action footage slightly. Avatar was projected at 1920 by 1080 for its 16:9 digital 3D version, but there was also a scope (2.35:1) digital 3D version made for the auditoriums set up for scope. This scope version probably had a horizontal resolution of 2048 (2K). The OT movies are, obviously, scope-only. Maybe Lucas thinks 1920 by 817 will still be considered good enough for a 3D projection in 2015, but he won't be getting around to OT until then, so who knows.