This is Ipomoea lacunosa of the family convolvulaceae:
This flower was found growing on the fence of the Veterans' Stadium parking lot fence in Jackson Mississippi in December of 2007. This plant is native to the lower 48 though not welcome in all states. It has the dubious distinction of noxious weed status in Arizona (I'll refrain from any puns) and Arkansas. It is found throughout the southeast as far north as Canada.