MPN Attribute
Also know as "Unique Product Identifiers" and "Global Trade ID Numbers", GTINs, for much more detailed information see the GTIN attribute page.

As mobile technology increases, so does how you need to work with it. Android and Iphones have now for quite some time been able to use the cameras to scan products bar codes and find the same product. Many of these devices use the Google Base API feed, to show products to the user, so if your products are on there and have the correct MPN, there is a healthy chance your product will come up in the results and you will make a sale.

Here is a the info regarding the UK specification and the US, German (DE), French (FR) and Japan (JA)

Other feed languages can be found here

There is also more info regarding Unique Product Identifiers here
http://www.google.com/support/merchants/bin/answer.py?answer=160161