Following directions on Android's main website to pull down sources, I'm looking at this command to initialize repo for the cupcake branch:
repo init -u git://android.git.kernel.org/platform/manifest.git -b cupcake
It doesn't seem to be possible using the "repo" script, but you can query the list of available branches using git:
$ git clone http://android.googlesource.com/platform/manifest.git $ cd manifest $ git branch -r
If you don't want to clone the repository just for this, you can see the branches on the web interface.