Had a note from Google which in summary said:
Starting May 3rd, 2011, we’ll require your product listings to include at least two of the three unique identifiers (Brand, MPN, GTIN(aka EPN)). 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/mercha...?answer=160161
With 1000+ products this will be a major task to hand edit the Google Product Data feed which I get out of Actinic every time we make a price change or add a product. How soon can we expect these identifiers to be added to Actinic (same as they added product condition) so it can be automatic.

Julian