From the IE Blog
...The best [IE7] add-on submitted between November 1, 2006, and February 9, 2007, wins a trip to Mix07 in Las Vegas and $2500 cash? Yep, we’re going to run a contest open to the entire world[1]. We think we’ve got some cool prizes lined up, like this Mix trip, or a first prize of $2000. We’ve even got a couple of Zunes for the honorable mentions.
For last year's contest we had our designers take a crack at remixing the MIX06 web site. Unfortunately the work was put off until the last minute, and the entry did not get submitted in time. However, we should have plenty of advance notice this time. This time will also be different because I can do the work myself and don't have to worry about getting set back by someone else's schedule.
More information and examples of how to build an add-on will be posted on the IE Blog. Official contest rules and information about the submission process can be found on this MSDN page. Good Luck!