I'm having the same problem and the UPC you provided did not work for me either. An ideas? My string is ...

http://xml.amazon.com/onca/xml?Service=AWSECommerceService
&Operation=ItemLookup
&SubscriptionId=<ID>
&ItemId=9780312278588
&SearchIndex=Books
&IdType=UPC
&MerchantId=All
&ResponseGroup=OfferFull,Medium
&OfferPage=1
&Condition=Used
&Version=2004-11-10

Thanks in advance for any help you can give me. The above string works for ISBNs with the change of removing the

&SearchIndex=Books and &IdType=UPC

clauses.