Many iPhone apps have the capability to integrate with Twitter, Facebook, and other social networking apps. It is possible yet to integrate with Google Buzz? I've checked the Google API docs for Objective C here
http://code.google.com/p/gdata-objectivec-client/
But I don't see any mention of Google Buzz.
The Google Buzz APIs are located at the link. As far as I am aware, it is not yet possible to publish directly to Google Buzz in any language, let alone in Objective-C. Although, if you run a thirdparty website, you can link your website to Buzz, and then publish to that website. Hope that helps.
There are currently no Objective-C client libraries specifically for Google Buzz. However, there's no technical reason one couldn't be written at this point, now that the REST API has been released. The documentation is quite extensive, and the OACurl cookbook should give you a pretty good idea of what's possible with the API and how to do it.
The API is not based on GData, however the Buzz API example app for the iPhone uses the Objective-C GData libraries in lieu of an actual Objective-C Buzz client.