With the official public release of Microsoft’s Internet Explorer 9 browser this month, an issue came to light for users on websites that used a previous version of the Cufón font replacement solution.

As explained by John Hrvatin, Internet Explorer program director:

The older Cufon library depended on legacy, non-standard IE behavior fixed in IE9. The IE9 behavior is correct (as in, IE9 now behaves like all other browsers) and won’t change so we’ve worked with the Cufon authors on a fix for the library. And as we find sites affected by this, we contact them to let them know there’s an updated version of Cufon to use.

So, it turns out, the font rendering engine Cufon used in IE8 will not work in IE9. If you are a developer that has used Cufon in the past, it is important that you update your Cufon files on your server to the latest version (currently 1.09i) to ensure that IE9 uses see the fonts on your website properly.
Luckily IE9 is still in its early stages and currently has below a 3% browser market share, but as more users upgrade, this will become more of a problem for developers that have not updated Cufon on their sites.