Hi,
may i ask what entity you try to map the Contact? you may use the GUIDID of each contact Vs The GuidID of the Serial Number.
try this one.. I already use this on migrating data in CRM 4.0
http://mscrmbulkupdatetool.codeplex.com/
Zosimo D. Recio Jr. :)
Zosimo D. Recio Jr. Microsoft Certified (Business Management Solutions specialist)