To reply to cinnamongirl first (since i'm sorting through thoughts in no particular order here) yes, Lilith was the first wife of Adam. She was kicked out of Eden for not wanting to be subserviant to Adam, and later had a line of children that were purported to be succubi/incubi (sexual vampires, basically).
Noah and his kids weren't directly incestuous... it was noah's son's and their wives that were on the arc, so it would have been a cousin situation by the third generation. Still incest and bad news for the genetics, but better at least than a brother/sister relationship.
As for there being other people elsewhere when Caine killed Abel and was sent into the wilderness, it makes sense-- if there were no other people that would try to kill him, the mark of Caine doesn't seem like it would be needed. I guess I always presumed that although Adam and Eve were the FIRST humans, there weren't the only ones created directly by god from dust, or ribs, or whatever.
On a side note, it is an interesting idea to think of the whole "made in the image of god" thing being tainted repetively by incest by Caine's kids, Noah's grandchildren etc. If it weren't for that maybe we would all still be able to float around using our god-like powers!
