This bug was confirmed by Microsoft after we opened a "Severity A" issue on it. The original response from the support guy was : "oh my god!" :) because this kind of bugs are really easy to track down in a superficial QA testing. Eventually
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Read the complete post at http://blogs.microsoft.co.il/blogs/itaysk/archive/2008/05/07/sharepoint-designer-data-form-web-part-context-menu-bug.aspx