<p>What actually happens to the file system when you do a Subclipse Share Project on an Eclipse project that was externally checked out from Subversion?  All the .svn folders are already in place.  I get an error when I try to Share Project the right way, and I'd rather not delete and re-checkout the projects from the SVN  Repository browser.</p>

Dunno exactly what happens within eclipse, I presume it does some funky stuff in the .metadata directory of the workspace.  That said, I would recommend the following to get eclipse to learn about the svn settings of the project:
Delete the project from the workspace (keep "Delete project contents on disk" unchecked)
File > Import... > General > Existing Projects into Workspace
Browse to the folder containing the original project(s) of interest
Import the projects into your workspace
This seems to have the side effect of subclipse noticing the subversion settings when importing the "new" projects into your workspace.