That's funny -- you're the only person I ever heard say they thought it was a joke. I'd always assumed he was dead serious, although that doesn't really make sense considering no one in the series had ever mentioned it before -- except in "The Wish". Didn't he say then there there *is* a great deal of demon activity in Cleveland? I don't recall, now.

This is another great thing about fandom -- you can take whichever interpretation you prefer, and run with it!