Doing some searching, I found quite a few complaints about the PhoneGap documentation, but eventually figured this out on my own.
when creating a project, a hidden .cordova folder is created with some project information. For whatever reason, this didn't go into the repository the first time. I copied it over to the checked-out version and fortunately, now Cornerstone sees it and will let me check it into SVN so other team members will not run into this issue.
If you get this error, there's probably a problem with the hidden .cordova folder inside your project folder. Either recreate the project in PhoneGap, or copy it over if you have another version of that same project.