Kreia is through and through a Sith. Before the Rule of Two is established, you can't really be "exiled" from the Sith Order. It's a kill or be killed business, and your not really initiated as your are in the Jedi Order. Saying she isn't Sith is like saying Darth Krayt isn't Sith. Her personality, her teachings, her beliefs, and her actions all point to her being a Sith. I've seen some people a while back on different forums argue that Revan isn't a Sith either, which is now known to not be true.
BW's TOR puts this in a new light; a lot of people thought of Revan as a "fake" Sith of sorts, due to Kreia describing the Sith in the Unknown Regions as the "True Sith". This is of course nullified by the fact that Revan and Malak were sent by the Sith Emperor to conquer the Republic, so for all intends and purposes they were Sith.
Yes, this also means that Kreia was dead wrong when she was hero worshiping Revan and proclaiming he never really fell to the dark side, but only did so for the greater good. Revan for the most part was a pawn; first he was a pawn of the Emperor, and then he was a pawn of the Jedi. Perhaps Revan had different motives in mind when conquering the Republic; he may have planned to turn against the Emperor. There is barely any evidence suggesting he would do such a thing besides the fact that the Sith betray each other all the time (see Malak vs. Revan). If Revan were going to betray the Emperor, it would have been for selfish reasons, not to "save" the Galaxy. Interesting to note that Revan was never really proven to be a strategic genius; beat the Mandalorians when he outnumbered them 5 to 1. Kudos.
It's also important to note that Kreia is a compulsive liar (Lord of Betrayal....). Anything she says should be taken with a grain of salt because she's a Sith.
<<< This guy spends way too much time on the TOR forums.