As long as you don't plan to release for a few more years I suppose. I mean, people are still using IE 6 today. It will be a while before IE9 1) comes out and 2) is adopted by a large number of users.
Fair enough, but I don't mind making lemonade with my low bandwidth graphics and mapping engine that doesn't require tile reloads on zoom. And it will work on the iPhone/iPad. IE support in a couple years is just icing on the cake.