Version 2011.2.915.40 does support IE9.
Could you please provide a live URL to the problematic page or open a support ticket and send a sample working project which demonstrates the problem?
By the way are you able to replicate the problem in the Default Example
The only workaround that I could provide without problem investigation is to render the page in IE8 mode using the following meta tag:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed