I love the fact that you can pin your blogs with Internet Explorer 9, but there are few…problems that arise when using Internet Explorer.
I’m not sure if this is only with WordPress.com or with self-hosted blog, but I noticed that dragging and dropping does not work in IE, unless you enable Compatibility Mode. This most likely means the blog is feeding IE users different code. I’d suggest using different code for IE9 users, as using Compatibility Mode messes up other parts of the blog/site, such as the top bar and any rounded elements.
FYI, the user agent strings for IE9 can be found here.
Since IE9 is still in beta form, you will need to use Compatibilty Mode for some sites that are not quite ready for IE9. With the final version of IE9, this shouldn’t be the case. If you’d like to report this to the IE9 development team, you can do so at http://connect.microsoft.com/IE/