It's a simple question, and the answer is out there on the net, but it took me awhile to find it. Eclipse launch configurations at the workspace level are stored in .metadata\.plugins\org.eclipse.debug.core\.launches.
I had assumed they were in org.eclipse.jdt.core and completely bypassed the debug package while searching around for it on my filesystem. Obviously that was a mistake!
Note that this refers to launch configurations saved by Eclipse as you work. You can also save your configurations to an external file, using the "common" tab of the run dialog, and saving to a "shared file".