I think its in Am. There clearly are parts of the song that seem to "resolve" on the C chord, but they quickly 'descend' back down: C, to B, to Am - except for the end of the song, which stays on the C chord. But overall, the song has more of a minor key 'feel' than a major.
But in thinking about this again, I can also see how one could argue the fact that since the song continually seems to 'resolve' on the C chord, that it IS in the key of C.
So.....your guess is as good as mine!!