By default there's an error in the TaxonomyPicker.ascx file SharePoint 2010 that has even not been corrected yet in SharePoint 2010 Service Pack 1. You can easily fix it yourself. When you're getting the following error in the Event Viewer on your SharePoint 2010 system:
Load control template file /_controltemplates/TaxonomyPicker.ascx failed: Could not load type 'Microsoft.SharePoint.Portal.WebControls.TaxonomyPicker' from assembly 'Microsoft.SharePoint.Portal, Version=188.8.131.52, Culture=neutral, PublicKeyToken=71e9bce111e9429c'.
Follow these steps to fix the problem: