To add to US Purchase's reply, below is the email that I received from google:
*Starting May 3rd, 2011, we’ll require your product listings to include at least two of the three unique identifiers (Brand, MPN, GTIN). In order to provide as much information as possible to our users, we need your help in matching your products to our "product pages", where users are able to view useful data such as product specifications or reviews. While these attributes will not be required for your data feed to process correctly, they may prevent your items from appearing in Product Search results if not included. For more details, including what specific identifiers you should include for each category of item, please see our Help Center at http://www.google.com/support/merchants/bin/answer.py?answer=160161
Starting June 6th, 2011, we’ll also require your product listings to include tax and shipping information to provide buyers with important price information. This information will be required at either the account or item level (in the “tax” or “shipping” attribute) in order for your items to process and appear in Google Product Search results. For instructions on how to include tax and shipping information, please visit http://www.google.com/support/merchants/bin/answer.py?hl=en&answer=160162
Please update your account settings or data feeds as soon as possible to ensure that you are complying with these new requirements by the dates specified above. For more information about these changes, please visit our Google Merchant blog at http://googlemerchantblog.##########################/2011/03/changes-for-google-product-search.html*