As we warned a few months ago, Google Merchant Center is going to start requiring unique identifiers for most products uploaded to Google Product Search (Google Shopping).
This requirement started officially at midnight last night, May 3, 2011. Google blogged it yesterday once more to notify Google Merchants of the change.
Although the requirement is now officially being enforced. Google said it is not being full enforced on all merchants. Google will begin rolling out the requirement now and by June 20th or after, Google will begin disapproving data product feeds without this required field.
Google's Ronald also posted this in the Google Merchant Center Help forum, where he said:
As we announced on March 14, and in emails to all Product Search merchants, today we will begin requiring unique identifiers for products in all categories except for apparel, custom, and one-of-a-kind items. These requirements are part of an ongoing effort to make Product Search better for our users. Our unique identifier requirements enable us to build detailed product pages to help shoppers learn about products and compare prices from across the web. These unique identifiers are only required for Google Product Search, and this policy will not impact Google Commerce Search or AdWords ad formats.
We will be gradually enforcing this policy over the next few weeks, with full enforcement in effect on June 20th. During this time, products that do not have correct unique identifiers may not appear in our product listings. If you have products that do not have correct unique identifiers, we recommend that you make the necessary changes as soon as possible.
Please note that in the weeks after June 20th, we will begin disapproving the feeds of merchants who have a significant percentage of items that do not include the correct unique identifiers.
Again, I'd add the unique identifier field asap, if you have not. But it seems you still have a month or so to add it until your feeds are disapproved.
Forum discussion at Google Merchant Center Help.