We provided full support for IE8
in Q1 2009 SP1 (version 2009.1.402)
and all subsequent builds of RadControls for ASP.NET AJAX support it. To fix the problem, my recommendation is to upgrade to the latest available build of RadControls for ASP.NET AJAX which is Q3 2010 SP1 (version 2010.3.1215).
the Telerik team