I've just finished my first release of an open source project for doing svn:externals -like actions/deployment with any combination of subprojects managed under different source control management systems.
I have several projects, some of which are managed under subversion, and some managed under git. What's worse, is each project has several subprojects (rails plugins, perhaps a vendor/rails), half of which are managed under git the other half under svn.
I really miss using the svn:externals feature of subversion for managing subprojects. svn:externals allowed me to have your subprojects be managed by a different repository than your main project. Doing a checkout/export automatically checks out/exports the subprojects. Doing a status will show you if any modified files in subprojects.
ext allows you to use this same workflow with any combination of source control management systems.
I have written a tutorial for ext at http://nopugs.com/ext-tutorial The source code is also available at http://github.com/azimux/externals
It should be pretty easy to install and play with and is quite feature-complete
This is my first ever attempt at contributing an open source project
to the community. Any advice/suggestions would be much appreciated 
I started this project out of necessity to get around the annoyances
of using git/svn together for managing and deplying projects with
subprojects, and think it could be really beneficial to other
programmers.
Thanks, Miles