The GData team just shipped JSON support for Google Base, Blogger, and Calendar feeds. Check out the updated documentation and samples, and the GData blog for further details:
"For those of you who have been trying to build client-side GData mashups but have been thwarted by the same-origin policy, we have some good news for you: you can now get public Base, Blogger, and Calendar feeds as JSON! This means that you can start displaying GData in your web page with a little JavaScript."