From the leaks the plot of the first two acts is pretty clear: it is basically an extended meditation on the description of the soul given in books 2-10 of Plato's Republic. The movie uses the character of Joe's "soul" to express his inner turmoil.
The plot concern's Jamie Foxx's character (Joe Gardner, a jazz musician) arriving at a city set up along the guidelines set forth in Republic books 2-4 (the city explicitly follows Plato's three-part caste system of craftsmen, guardians and complete guardians/rulers).
Now at first, Jamie Fox's character sees the Platonic city as a truly just city: growing up as a "fantastically gifted saxophonist" on the mean streets of Harlem in the 1920's, he actually agrees with the notion that the best be educated more than the rest (there is a funny line of dialogue where one of the guardians says to him "Have you ever tried to teach an unmusical person an instrument?" triggering a series of 5 second flashbacks of terribly bad music students Joe has had.) Joe also remarks on the fact that the society is free of racism (the caste system is purely meritocratic) and on the noble behavior of the guardians. He is particularly struck by the egalitarian education of the woman guardians, (so different from his home town) and sparks up a friendship with a female guardian named Diotima (Tina Fey's character). The first act ends with him madly in love and declaring to the sky that he is "never going to leave!"
The second act, however, introduces the major conflict of the movie: due to the laws of the city Joe is not able to perform Jazz. Music is tightly regulated in the city to prevent the corruption of the youth. Joe is thus incredibly torn (which we see explicitly: his soul is shown splitting in two). The second act is a dramatazation of this inner struggle. Don't know the 3rd act.
Overall it looks like a pretty bold and original concept for Pixar, so I am excited to see how it turns out.