Just FYI, I was getting a 401 from my Captcha control (using SharePoint + IIS6) when it accesses the Telerik.Web.UI.WebResource.axd handler. This is because my SharePoint site is not anonymous at the top level, but only at specific sub-site levels.
Adding this to the Web.config file seemed to work:
<location path="Telerik.Web.UI.WebResource.axd">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>
Adding this to the Web.config file seemed to work:
<location path="Telerik.Web.UI.WebResource.axd">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>