If you run into the following error while adding a project into Eclipse from existing source using the New Project dialog,
Cannot create project content in workspace.
then you are very likely trying to create a new Eclipse project that contains Eclipse metadata files from a previous Eclipse project created at the same location. The fix for this is to import this pre-existing project into your workspace using the Import, General, Existing Projects into Workspace option.
Hope this helps someone out there :)
D