As a Windows phone user myself, I will be happy if you can support IE. I am not asking you to make decisions based on IE support but if you can support IE with only little tweaks to your iPhone/Andoid codebase, I would recommend you to do that. If it works in desktop IE9 it will work in mobile IE9.

I've had some experience with trying to make things that work on iOS/Android work on WP7 and it's nearly impossible. For starters mobile IE does not implement tap/touch handlers; which is a dealbreaker for mobile web apps.

I think that may have been his point. There's a small group of people who insist that Microsoft's mobile devices will take off. Those will run IE. I know I've been pushed away from using certain libraries for that reason.