^^^
post above pretty much nailed it...
Yeah, usually the key signature will tell you the sharps and flats, and through this the key (although it doesn't tell you if the piece is major or minor). If this "Key: C" indication is at the top of the page, it could be simply to clear up whether the piece is in C or Am, assuming there is nothing in the key signature before the clefs. If the "(sometimes Cm, D, A, etc.)" is part of that first indication then I suppose they're just preparing you for key changes within the piece, so get ready for 3 flats, 2 sharps, and 3 sharps, as well as other possible changes.
As for finger placement, keys very rarely have anything to do with that. A piece starting in C major could easily begin with a first inversion triad (E, G, C), and although it's possible to play that with your thumb on the C below the E, it's sort of awkward. Why use 2, 3, 5 fingering when you could use 1, 2, 5?
All in all, i don't really think it's relevant to how you play the piece, just moreso for your own knowledge. I wouldn't worry about it. If I'm interpreting how this is laid out wrong, feel free to correct me, and I'll help where I can.
Edited by Meddle71 - July 28 2009 at 01:55