Does section of a Web.config file look familiar?

Matt Gibbs has a workaround...

<!-- The tagMapping section is for a CTP version of Ajax and must be commented out after installing the Ajax release version -->

<!--<tagMapping>

<add tagType="System.Web.UI.WebControls.CompareValidator" mappedTagType="System.Web.UI.Compatibility.CompareValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add tagType="System.Web.UI.WebControls.CustomValidator" mappedTagType="System.Web.UI.Compatibility.CustomValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add tagType="System.Web.UI.WebControls.RangeValidator" mappedTagType="System.Web.UI.Compatibility.RangeValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add tagType="System.Web.UI.WebControls.RegularExpressionValidator" mappedTagType="System.Web.UI.Compatibility.RegularExpressionValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add tagType="System.Web.UI.WebControls.RequiredFieldValidator" mappedTagType="System.Web.UI.Compatibility.RequiredFieldValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add tagType="System.Web.UI.WebControls.ValidationSummary" mappedTagType="System.Web.UI.Compatibility.ValidationSummary, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

</tagMapping>-->