Prequel Watch has a great article and several spy reports pertaining to Episode III.

"Final costume and wardrobe tests are underway."

Spoilers - Highlight to read: "A classic trilogy looking Stormtrooper has been spotted. Well, at least it looks more like the troopers from the originals than that of the clones from EPII anyway."

"Hayden Christensen has been on set, working with stunt coordinator Nick Gillard. He is masterful with the saber already. Ewan McGregor will report soon and the two will begin blocking THE DUELS."

"The crew is pretty much done with completing what has to be the largest practical set built for the prequels so far."

Spoilers - Highlight to read: "Darth Vader remains the most tightly-guarded and closely-kept secret of the film so far. The current crew knows almost nothing about the character's appearance and role in the film (other than he will be in it). Not even the prop or costume people know much at this point (or if they do they aren't talking). So it might be safe to say that Lucas is taking a "Return of The Jedi" like approach to the character and how his appearance will be dealt with as far as secrecy."

"Shooting begins in just over 3 weeks. Ian McDiarmid is said to be scheduled for the first day's shoot."

There are some great updates here, and their source has supplied information in the past that has been entirely reliable so far. But for now I would consider these to be just a rumor.