I've found several web pages that describe how to use git submodules to install plugins, but it seems that those pages are missing something (or more likely, I am) .
In a nutshell, they advise replacing
$ script/plugin install git://path_to_spiffy_plugin
$ git submodule add git://path_to_spiffy_plugin vendor/plugins/spiffy_plugin
It seems to me that these are not exactly equivalent... aren't there installation scripts that get run when one runs "script/plugin install"? Is it as simple as manually looking for a vendor/plugins/spiffy_plugin/init.rb and running that script in Ruby? Or does it need to be run using (somehow) script/runner, so that the proper Rails environment is set up for the script.
Are there other things that script/plugin install does?
I notice that there is a -x option for script/plugin install, but that is specific to subversion. It doesn't work with git.
Finally, for anybody who stumbles across this post in the future, I believe that the equivalent to the "-r" option for script/plugin install is "-b" for git submodules add. e.g.
$ script/plugin install git://github.com/ewildgoose/render_component.git -r rails-2.3
$ git submodule add -b rails-2.3 git://github.com/ewildgoose/render_component.git vendor/plugins/render_component