Looks like a problem with rails trying to autoload a missing constant that's already loaded. You don't mention the version of Radiant that you're using, but the simplest fix is probably to load up the version of Radiant that you're not using (rc 2 or .91) with a default template and using RVM to create a dedicated gem set for testing purposes. Some of the thorniest problems in configuring Radiant are versioning issues involving gem dependencies. Things will get better when Radiant moves to Rails 3. If you're not using RVM, it can be very helpful as it allows you eliminate the gems and gem versions that you might have installed globally for other applications.
You could also try posting in the paperclip group (http:// groups.google.com/group/paperclip-plugin).
