Great thread Kevin so many interesting posts by all in here thanks for starting it and for all who posted.
Thank you, and I too thank everyone participating! Great to read different takes on the same subject.
As for the final scene- I'm with Indy Blues in that both Mac and Childs are human, however are still suspicious of one another.
I always wondered what was behind Mac's final chuckle after Childs takes a swig of the whiskey-
I'm of the conclusion that before Mac hands him the bottle he still believes Childs is The Thing, but after is slightly relieved (because for whatever reason, Childs taking a drink convinces Mac that he is human). And now the two can relax in peace (Mac rests his head), knowing that they have saved Mankind (albeit sacrificing themselves in the process).
There is also something of a final "truce" between these two characters who have been at each other throughout the film. Childs questions every decision Mac makes, going so far as trying to kill him at one point (locking him outside), and even "rubbing it in" that he killed Clark who was human after all.
Mac threatens to kill Childs when he refuses to be tied up for the blood test (and was probably a second away from actually going through with it), and is still suspicious of him after the Outpost goes up in flames.
However the end sequence not only demonstrates that they have prevailed against the Thing, but have reconciled. Outpost 31 was torn apart not only by the Thing, but by our natural tendancy toward paranoia and fear. Mac and Childs coming to terms is another "win" so to speak against the Thing who played on Man's basic fears so well.
In essence this is the "happy ending."
Kevin