Google now also requires merchants to include either Universal Product Code(UPC),Manufacturer Part Number(MPN), or European Article Number(EAN) to all the products in the feed. Google has stated that they will remove feeds that do not meet these requirements.
If for some reason your products do not have these identifiers available, you can submit an exemption request by visiting the following page :
http://www.google.com/support/merchants/bin/request.py?contact_type=unique_id_exemption