There is a plugin declaration in the application template runner that can specify either an SVN or git repo. In the case of git repos, it can also specify whether to clone or use a submodule. However, to specify a branch or revision, you have to add command-line params to the repository spec:

add stable branch as a git submodule

plugin 'x', :git => '-b stable git://github.com/x/x.git', :submodule => true

add stable branch as a git clone

plugin 'x', :git => '-r stable git://github.com/x/x.git'

add rev 1234 as a subversion working copy

plugin 'x', :svn => '-r 1234 svn://svn.rubyforge.org/x/trunk'

I've attached a patch that allows the user to specify a git branch or svn revision using options:

add stable branch as a git submodule

plugin 'x', :git => 'git://github.com/x/x.git', :submodule => true, :branch => 'stable'

add stable branch as a git clone

plugin 'x', :git => 'git://github.com/x/x.git', :branch => 'stable'

add rev 1234 as a subversion working copy

plugin 'x', :svn => 'svn://svn.rubyforge.org/x/trunk', :revision => 1234