Ronald from Google in the Google Merchant Help help forums announced changes are happening to Google's product feeds.
Specifically, if you submit products in the categories of Electronics, Books, and Media in the U.S. you will need to pass along more product attributes or the products won't display in future searches. For these products to show, you will need to include the unique product identifiers, such as MPN, Brand, GTIN/UPC/EAN or ISBN.
If you do not include them, the feeds will continue to work, but the products that require that data will no longer show up on Google Product Search.
This requirement goes into affect "late August" and you can learn more about this at the Google Help section.
Forum discussion at Google Merchant Help.