We announced full support for IE 8 browser with the Q1 2009 SP1 release of RadControls for ASP.NET AJAX:
With previous versions of the suite you may consider setting the compatibility meta tag of the browser to simulate IE 7 in order to avoid unexpected discrepancies.
The alternative option is to migrate to the latest version of the controls following the instructions from here
. Note, however, that there are some breaking changes in the Q1 2009 release you should be aware of - they are listed here
the Telerik team