>>At a guess I'd try declaring loDocumentDates.dtDocumentDate as a nullable type and just not setting it if loRow("CloseDate") contains an empty date.....
>
>When doing so, we get:
>
>Server was unable to process request. ---> SqlDateTime overflow. Must be between 1/1/1753 12:00:00 AM and 12/31/9999 11:59:59 PM.
>
>So, they are validating. My guess is that even if we go low level, this won't work as we would probably obtain the same.

Did you check the XML that was sent? This sounds as if my suggestion should work:
http://msdn.microsoft.com/en-us/library/ybce7f69(VS.80).aspx
Snip:
"When serializing objects into an XML document: If the XmlSerializer class encounters a null reference for an object corresponding to an XML element, it either generates an element that specifies xsi:nil="true" or leaves the element out entirely, depending on whether a nillable="true" setting applies."