These are the types of errors I get from it trying to fetch the submodules:

Fetching upstream changes from git@github.com:absperf/repo.git/subrepo/.git
[subrepo] $ /usr/bin/git fetch -t git@github.com:absperf/repo.git/subrepo/.git +refs/heads/:refs/remotes/origin/
ERROR: Repository not found. Make sure you include the .git, e.g. git@github.com:defunkt/ambition.git
fatal: The remote end hung up unexpectedly
ERROR: Problem fetching from origin - could be unavailable. Continuing anyway
ERROR: Could not fetch from any repository
FATAL: Could not fetch from any repository
hudson.plugins.git.GitException: Could not fetch from any repository
at hudson.plugins.git.GitSCM$2.invoke(GitSCM.java:675)
at hudson.plugins.git.GitSCM$2.invoke(GitSCM.java:642)
at hudson.FilePath.act(FilePath.java:753)
at hudson.FilePath.act(FilePath.java:735)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:642)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1038)
at hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
at hudson.model.Run.run(Run.java:1258)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:129)

Prior to the 1.0 release of the git plugin I still got these errors, but the build continued despite them and I could handle updating the submodules myself. Now the plugin is useless since most of my projects have submodules.