We were having trouble adding a user to a site collection.  Every time we hit the search button or clicked the check userid button the system would just sit there and spin. I added the following section of the web.config to resolve the issue.

</compilation>
  <pages enableSessionState="false" enableViewState="true" enableViewStateMac="true" validateRequest="false" pageParserFilterType="Microsoft.SharePoint.ApplicationRuntime.SPPageParserFilter, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" asyncTimeout="7">
    <namespaces>
      <remove namespace="System.Web.UI.WebControls.WebParts" />
    </namespaces>
    <tagMapping>
      <add tagType="System.Web.UI.WebControls.SqlDataSource, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" mappedTagType="Microsoft.SharePoint.WebControls.SPSqlDataSource, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
    </tagMapping>
  </pages>
<siteMap defaultProvider="SPSiteMapProvider" enabled="true">

The error that we were getting in the trace logs is similar to below.

via Solien Technology