I am testing using fully patched IE 9 and Windows 7.
I was opening my radwindows differently than you and i was able to reproduce it in a small project: http://www.4shared.com/file/VZYTwqL6/MetaTagIE7.html
Open Default.aspx and click the link to open the radwindow. In the popup, if you click and hold the button hopefully you will see the issue I'm having.
I am going to remove that <meta> tag from my pages as the end user will likely be using IE9.
FYI, in addition to the button appearance, that <meta> tag also caused other problems, mostly with layout.. padding and spacing between elements.. different widths of elements.. that kind of thing. Weird thing is it only seemed to get whacky with controls inside a radwindow where the parent had