Okay, this is going to be a very long post, the website this was on has been deleted but it was in an editorial on rpgamer so here goes:
"fact 1: i believe that sorceresses do not age. why? because it never in the whole game speaks of any sorceresses dying. it speaks of adel being from the far past, yet she is still alive. also, present day cid looks a lot older than present day edea. it is possible cid was as young as edea looks when edea received her powers, making cid age but not edea. if it is true sorceresses do not age, rinoa, being a sorceress could live on and become ultimecia."
"fact 2: it is stated in the game that in order for a person to become a sorceress, a dying sorceress must pass her powers onto them, in order to die in peace. however rinoa does not become a sorceress in this way. she becomes a sorceress after you have fought edea in galbadia garden. true you have to beat edea, but she's not dying, nor does she die, it is never explained how rinoa became a sorceress."
"fact 3: now i know what you're thinking, why would ultimecia want to kill squall if she's rinoa? well there are many indications why. i believe what happened to rinoa was, she aged and squall didn't, and eventually he died and she lived on. unable to take the pain she became evil. it might sound a bit wayward but read this quote from ultimecia just before she dies: "reflect on your..." "childhood..." "your sensation..." "your words..." "your emotions..." "time..." "it will not wait..." "no matter..." "...how hard you hold on" "it escapes you..."time will not wait, no matter how hard you hold on it escapes you? this backs up my theory of squall dying. she wanted to hold onto him but he died."
"fact 4: on disc 3, squall says to rinoa at the field next to the orphanage "i'll be here waiting for you i promise ... etc..." and suddenly the whole intro makes sense. now is it just a coincidence that ultimecia's castle is placed exactly next to this? you have to ask yourself, why the hell would ultimecia park her castle there when she has anywhere in the whole world to put it? she is waiting for squall at the field..."
"fact 5: now you might think, why would ultimecia kill squall when you fight her at the end, surely this is what she'd have wanted, to be reunited with squall. this is a valid point, however she does not recognised squall is squall, this is proven when edea explains who she is, squall approaches ultimecia and she says "who are you!?" (or something along those lines)."
"fact 6: now this is a big indication that its true: rinoa 'borrows' squall's griever ring, yes she does get zell to make a copy, but she never gives squall his back, this is proven at the field when rinoa says "i have this dream, i get all dressed up and put on your ring... etc...".just before you enter galbadia garden, you get to name the ring, (the default is griever). ultimecia has a gf.......called griever, (or whatever you name the ring). if you remember, you get doomtrain out of solomons ring. its possible griever is gained from squall's ring, which ultimecia would have if she is rinoa."
"fact 7: ultimecia has black wings, rinoa has white wings. there is a lot of emphasis placed on rinoa's wings (on her costume, and limit). black = evil, white = good. it would be a natural progression. that is backed up by these shots taken from the ending. it appears rinoa's wings are changing from white to black."
"fact 8: there are 3 fmv shots of ultimecia in the game, all are at the ending, and last less than 1/10th of a second each, barely visible (did you notice them?). why would square go to all the trouble of making the fmv if no one were to see it? square aren't stupid, they wouldn't waste time and money for nothing."
"fact 9, the biggest: now i knew myself i would never believe it unless i saw that rinoa looked like ultimecia, and from what i could remember she didn't. however, look at these pictures i have superimposed onto one another (they aren't exactly right because the pose ultimecia is doing, rinoa doesn't do, so i found the nearest match (see the bottom of this page for pictures)"