My brief and probably, judging from the length of Squall's response (which I don't have time to read tonight), useless contriubtion to this thread:
I always assumed the "Sephiroth" after which the party spends the first disc of the game chasing was in fact Jenova assuming a shape that would have more resonance to Cloud, since the first time he appears is after Jenova escapes from her prison. I've never been particularly clear on how she escapes, though, or if I ever was I've just forgotten. It's probably the latter actually.
