Wouldn't the smart thing here be to design an app that works in ALL (at least modern) browsers?
To begin with IE only shuts out all Linux users, as well as most Mac users (which will probably be using a Geckobased or Opera browser insted of downloading MS bloatware to their Mac).
Also, what happens if/when AOL switches their 40 million users over to Gecko based browsers instead of IE? (AOL owns Netscape and is no longer bound by a exclusive use IE as engine agreement)?
Relying on (as opposed to using) proprietary code to present content on the net is really not a good thing in the long run.