I recently read this on the Final Fantasy VIII FAQ post in the Final Fantasy VIII section of the forums:

Question 4:
Are Rinoa and Ultimecia the same person?

Answer:
There is no clear answer. There's evidence that suggests they are, and clues that they're not. It's a widely argued issue, but one that's nearly impossible to resolve satisfactorily.

This is widely called the 'R=U' debate.
Thanks to twobitmage for recommending that a definition of 'R=U' be included in this FAQ.

New information:
The FFVIII Ultimania guidebook is an official publication containing detailed background information on Final Fantasy VIII. According to this guide, the sorceresses in FFVIII have normal human lifespans. As Ultimecia lives "many generations in the future", it is not possible that she and Rinoa are the same individual.
So much for that theory
Heartfelt thanks to Squall of SeeD for posting information from the Ultimania guide and putting an end to the most annoying debate in Final Fantasy history.
Even though the above information may be true, Rinoa may have become Ultimecia in her lifetime and condense time to an earlier point when she was still Rinoa for the purpose of controlling her entire lifeline of existence. Besides, when one involves the alteration of space-time, all normal "lifespans" and "periods" go right out the window.

What do you guys think?