Contrary to initial reports from the UK, and even the corrections of them quoted at IMDB, Lucasfilm has confirmed Natalie Portman was indeed invited by C4 and some time ago her representatives personally advised Steve Sansweet she would be unavailable as she would be out of the country.
Earlier posts on the topic:
Apparently Natalie Portman is disappointed at not being invited to Celebration IV. According to her: "It's upsetting. You would have thought, being the queen, I would have at least received a letter."
UPDATE: Natalie Portman's reps are now upset at reports that she was upset about not being invited to CIV. Got all that? Check it out here at IMDB.com. Scroll down to the middle of the page for the story (Thanks to Scott from TFN)