Error: The type Microsoft.SharePoint.Portal.Analytics.UI.ReportViewerMessages, Microsoft.SharePoint.Portal, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c does not implement IReportViewerMessages or could not be found.
Cause: Look in your SharePoint web.config file you'll see
<add key="ReportViewerMessages" value="Microsoft.SharePoint.Portal.Analytics.UI.ReportViewerMessages, Microsoft.SharePoint.Portal, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />.
Fix: I looked at the Microsoft.SharePoint.Portal.Analytics Namespace and couldn't find a UI.ReportViewerMessages, so I just removed it from the web.config and I haven't experienced any issues.
You can also remove the setting from your ReportServer web.config's just add:
<appSettings>
<remove key="ReportViewerMessages" />
</appSettings>