I tried to checkout the GData for Obejctive-C client (found the link from here: http://code.google.com/p/gdata-objectivec-client/source/checkout), but it kept telling me that revision is non-existent. I also tried different tools and methods. But none of that worked. If you have any idea about this problem, please do let me know.
Wrong URL. open Terminal and do this:
svn checkout http://gdata-objectivec-client.googlecode.com/svn/trunk/ gdata-objectivec-client-read-only