I started using Font Squirrel a while ago to embed custom font faces into websites. After upgrading to Windows 7 I encountered a heavily documented issue about IE 9 and @font faces.
There can be problems maintaining backwards compatibility for @font face within different versions of IE. Some versions support .eot font face, others support the new .Woff format. The ordering of the CSS declarations can have a massive impact on how this works.
Rather than testing this within the site I'm having issues with I thought I'd put together a test page to ensure I was getting accurate results and could easily test it in a few browsers.
This is all based on the default scripts provided by Font Squirrel and a whole lot of reading on Paul Irish's blog:
If you are considering doing anything at all with @font-face I'd suggest you go read a few of Paul's articles as he has most of the angles covered.
To view the test page follow this link: IE9 Woff support test page