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SELECT pr.String AS GUID
FROM tblProperty pr, tblPageDefinition pd, tblPage pa
WHERE pr.fkPageDefinitionID = pd.pkID
AND pr.fkPageID = pa.pkID
AND pa.Name LIKE '%(The page name here)%'
AND pd.Name = '(Name of the XForm property here)'
Take this value and insert into the default value text box on your page type in admin mode.
Dirty, yes. But I found no better way of doing this.
Regards,
/Marten