From the SRD @ http://www.wizards.com/d20/files/srdspellsp.rtf under Polymorph Self
Natural abilities include armor, natural weapons, and similar gross physical qualities. A body with extra limbs does not allow a character to make more attacks (or more advantageous two-weapon attacks) than normal. Natural abilities also include mundane movement capabilities, but not magical flight and other magical forms of travel. Extremely high speeds for certain creatures are the result of magical ability, so they are not granted by this spell. Other nonmagical abilities (such as low-light vision) are considered natural abilities and are retained.