Kenpachi already has achieved shikai. His sword is permanently in shikai form just like Ichigo's. However, Kenpachi is still yet to achieve Bankai <---- Not true, kenpachi needs to know the name of his zanpaktou, a perquisite for shikai
http://bleach.wikia.com/wiki/Kenpachi_Zaraki If you scroll down, you will find that Zaraki actually does have his sword permanently in Shikai, despite not knowing its name. This is due to his high spiritual energy always pouring out.