First, there will be no Ep. III on VHS, which is a shame for those VHS box-set enthusiasts/completionists.

Second - Ep. III will come out first as a stand alone. A box will be available to order to house your prequel trilogy. Details will likely come with Ep. III DVD.

Third, there will probably be a couple of trilogy re-releases, after the initial Ep. III craze dies down. Then, we're also likely to see a 6 movie set.

Fourth (and hopefully finally), there will be the super-duper, 6 movie (10 disc or more?) HD-DVD version.