I accidentally hit Source->Externalize Strings... on eclipse and now hundreds of string literals are stored in a properties file. I want them back scattered about my code. How can I do this without re-entering them?

If you are using Eclipse in a project with central code management and versioning just revert to an earlier version.
If not, depending on how you're set up, you should be able to right-click in the editor window for the affected file(s) and select "local history" to restore your source from Eclipe's local backup versions.