Or rather, an alternate dimension/universe version of Zelda (or, as Animateur puts it, "a different form of the same being"). I've seen this discussed here and there but not in detail, and I didn't find a thread for it so I apologize if there is one. Anyway, here are some reasons why I think this:
1. Physical parallels--their facial features are similar (in Midna's true form, obviously) and Zelda's cloak sort of resembles Midna's.
2. Their theme music has an inverted melody structure (With Zelda's theme using a middle note, a higher note, then a lower one and Midna's doing the exact opposite).
3. They're both princesses, and I think Midna could be a representation of what Zelda might have been like if she were under different circumstances; i.e. her people were banished and oppressed. Both care about their people above all others and it affects their outlook. Plus, Midna's sarcastic, troll-y personality is similar to Tetra's.
4. Some in-game symbolism; such as the Twilight Mirror itself with Midna being Zelda's 'reflection' (some people think Zant is Link's, but I'm not nearly as convinced as I am with the Midna/Zelda parallel). A more minor one is the way that Zelda rides on Epona with Link at the end somewhat mirrors the way Midna rides on Link's back when he's in wolf-form.
5. Their names are similar; two syllables, ending in a, set off with a d consonant.
6. The official art seems to have some implications;
http://www.zeldawiki.org/images/thum...x-Twilight.jpg this picture alone could be a piece of supporting evidence, but especially when compared to this one:
http://www.zeldatwilightprincess.be/...llpapers/6.jpg
Sorry if this seems way too obvious or talked-about, but I was just curious about other member's thoughts on this.