I've read plenty of reasons why she isn't, a few why she might be, but ultimately a lot about what makes Twin Peaks so interesting is the many different angles to approach a theory. Basically, I have two major points and a minor point to make about why she is, feel free to tear them apart in the comments.

1) In Fire Walk With Me, on the very night she dies, Laura tells James "Your Laura disappeared... it's just me now." This is also during a very erratic conversation between the two. It's easy to say "she was on coke" but her behavior seems more in line with Tulpa Diane's. Tulpa Diane seemed to obviously have a hidden motive, but at the same time she also seemed confused about her purpose, otherwise I don't see why she would have admitted that Cooper "raped her" when we now know she's not the real Diane, which means the Tulpa herself wasn't raped, but the memories and experiences of Diane still remain. It would also explain Carrie Page. If Carrie Page is in fact the original Laura Palmer, then Tulpa Laura would have replaced her, with Laura "disappearing" into another dimension.

2) Laura seemed to be living a double life. While this is definitely a thematic element to the show, it seems highly unlikely that Laura was able to do to the following:

Go to school full time, tutor Josie and Johnny Horne, participate in Meals on Wheels AND strike up a friendship with a shut-in, have two boyfriends, work at the perfume counter, work at One-Eyed Jack's AND be a free-lance escort that sometimes goes out of town for clients. I highly doubt one teenager could accomplish all that. Also? She was at home in bed on plenty of nights that we saw in Fire Walk With Me, which goes in line with the parental abuse story arch.

3) Remember how even before The Return, thorough the Secret History, we start to learn that Annie's existence is suddenly called into question? The main consensus seems to be she was a lodge entity of some kind, with her melding with reality to "fit" within it. When she dies, her place in reality seems to cause memory issues. This would "fit" with why suddenly the her murder becomes the "disappearance of Laura Palmer"... where'd she disappear to? Well, maybe she's Carrie Page now?

The only major point against these is that Laura in the Red Room tells Cooper that Laura Palmer is dead... but at the same time, she confirms that she IS Laura Palmer. Leland wants Cooper to "Find Laura" and that's exactly what I think he does in the end.