omg!! Kakashi has Mangekyou Sharingan!!! OMG!!
Maybe obito is not dead and he went back to kill him THUS obtaining the Mangekyou Sharingan!!
Or He feels guilty that he his best friend died because of him and subconciously think he killed him!Thus Obtaining the mangekyou!
or maybe his blood somehow got mixed with the Uchiha clan blood and became the Sharingan!

either that or he just changed his contacts :P