I think we all knew that the 31st December date was just a placeholder, but some advance warning would have been courteous on Zavvi's part! I certainly did not expect to randomly receive shipping information when as far as we're all concerned, a release date had not been confirmed yet!
At any rate, Zavvi should have contacted customers first to inform them of release date changes, but this seems to be something they don't seem to care about. Not everyone has the time to keep checking websites and forums every single day on the off-chance that release dates may have changed since the last check. It's the retailer's duty to inform their customers of important information such as this, and in that respect Zavvi have failed miserably on more than one occasion!