The git SCM plugin should provide the ability to automatically retrieve all submodules recursively. Developers often setup their repos to have a recursive submodule structure. It also doesn't make sense to have to manually enter git submodule update commands in the build shell commands just to initialize things properly.
Recursive submodule retrieval (update) can be easily done using the '--recursive' option which is available in git 1.6.5. This option should be disabled by default.
I've attached a patch that will implement this (see recursiveSubmodules-patch.diff). This patch was applied to commit 6c842954608. It also contains code to recursively clean the workspace's submodules if the 'Clean after checkout' option is selected.