I extracted asset_copier from an existing Terralien project. It fixes this problem beautifully. Once installed, it keeps the assets in your Rails project synchronized with your plugin. It even deletes assets from the Rails project that have been removed in the plugin.
To learn more, check out this post. If instant gratification is more your thing, install the gem directly and apply it to your target plugin:
sudo gem install pelargir-asset_copier --source=http://gems.github.com cd ~/some_rails_project script/generate asset_copier some_plugin