Symptoms
Google is no longer accepting your site's Product Search feed upload.
Cause
Over the weekend of 6/25/2011, Google made an (apparently unannounced) change to the Google Product Search feed requirements. The 'Id' field, which was previously recommended, is now required. Note that this change is separate from the other recent changes documented here.
Solution
Customers will need to make a very small, simple modification to their Google feed to adapt to this change:
1 - Open the 'feed.googlebase.xml.config' file in Visual Studio, or any text editor. That file is usually located in your site's {root}/admin/XmlPackages folder.
2 - Find the <item> line within the <xsl:template match="Product"> section of that file.
3 - Immediately after that line, add these 3 lines:
<g:id>
<xsl:value-of select="productid"/>
</g:id>
4 - Save the changes, and upload that file back to your site.