There were no apps available for the giant brick-like cell phones when the last Royal Wedding happened, but fear not, there are plenty to go around for this one.
Network World lists a whole pile of apps you can get for your i-device, but most of them seem to basically do the same couple things - countdown, and have a collection of photos / videos / information.
Just a few listed include:
Royal Wedding, from Neon Play, which focuses on mobile games for iOS. Free for iPhone, but the really good stuff -- 100 "exclusive photographs from one of the world's top royal photographers, Ian Jones" -- is only available as an in-app purchase. The rest is fairly routine, and static, data about Will, Kate, Westminster Cathedral, etc.
Royal Wedding, from AppSNow.co.uk.; $1.99, for iPhone. Version 2.0 promises "MASSIVE UPDATE!" which focuses on interactivity and live feeds: It pulls wedding news from a pack of British newspapers, and a selection of blogs. It promises a "complete social network integrated" just for app users: You can "favorite" and comment on stories, see what's being read by others, and it has tweets from Clarence House, the official residence of William, his brother, Harry, and his father and stepmother, the Prince of Wales and the Duchess of Cornwall.
Royal Wedding Countdown, from Innovative Video Solutions; $2.99. The name refers to the main screen which shows you the time remaining -- down to the second if you're a true Royal Celebrity Obsessive -- before The Wedding begins. The support page is mainly devoted to pleading with users not to write a negative review if you encounter a problem but send them a crash report instead. Good luck with that. Version 2.4 offers an "updated user interface" but doesn't say what's changed, and adds, somewhat belatedly it seems, a link to the official wedding website.
Check out their site for more, or do a quick search in the iApp store for "royal wedding."