According to ComingSoon.net, the Watchmen footage which was shown at San Diego Comic-Con is now officially online and can be watched for free at iTunes.
According to MarketWatch, also available starting December 9 on iTunes and the Apple App Store are: the Essential Watchmen app for iPhone and iPod touch; multiple behind-the-scenes video journal podcasts; film clips; trailers; and other features exploring the creation of the movie.
Source: ComingSoon.net, MarketWatch.