From the Slashdot summary:
QuoteProtocol Buffers is "a language-neutral, platform-neutral, extensible way of serializing structured data for use in communications protocols, data storage, and more."
As someone who has mucked extensively with XML (and a bit with JSON, which is supposed to be a "stripped down" version of XML) I have a lot of interest in seeing how this develops. Basically any clean way of pushing large chunks of data between components like services is always a pain and can be artificially slow. This stripped down API sounds like a great innovation and hopefully has been put through its paces already by Google.
At any rate, I will be playing around with this stuff when I get the chance.
P.S. BTW, the following APIs are supported: C++, Python and Java
I forgot to add the links:
http://tech.slashdot.org/article.pl?sid=08/07/08/201245
http://google-opensource.blogspot.com/2008/07/protocol-buffers-googles-data.html