WordPress.com's dashboard / notifications told me to upgrade IE but I have the latest (for Win7). Turns out the compatibility button was selected. Upon deselecting it,t he warning went away and the site looked fine. The problem is that if WordPress.com works with IE9 you need to add the META tag X-UA-Compatible with a value of "IE=9, IE=10" or just "IE=edge".
See http://msdn.microsoft.com/en-us/library/ie/hh771820(v=vs.85).aspx for more information.
Blog url: http://lifeofheath.wordpress.com/