Is it "damn thing" or "damned thing"?

When used as adjectives, damn and damned are synonymous. I would suggest that damn is more common, but that's based solely on anecdotal evidence and the fact that it's slightly easier/faster to say which is key in situations where it's appropriate :P

I agree with Dusty in the main, but damned can be said for emphasis and seems more deliberate and harsher than damn. When John D. Rockefeller said "the public be damned" it had an especially contemptuous feel to it. When the Duke of Wellington, threatened with publication of embarrassing information about himself, told the would-be blackmailer "Publish and be damned," it had a defiant, lordly ring to it.

You can say damn in a friendly way, but damned goes that extra mile.