Means generally the same as imperfect, just spelled incorrectly, thus furthering the meaning of the word. If used correctly, it could be used as and extremely awesome myspace name i.e.: PaigeUnperfect™
Girl 1) She's such an unperfect whore, I can't believe he's dating her.
Girl 2) Do you mean imperfect?
Girl 1) No, unperfect suits her more. *glare*