If you don't read or watch Bleach, this post will seem like nonsense to you. Otherwise, read on.
Wikipedia cites page 258 of the Bleach Official Character Book SOULs for this fact. The page can be downloaded at Ju-Ni. (Ju-Ni is a very good Bleach scanlation group, although they do have other projects too. Membership is free, so don't complain.) For those who don't like to register on sites they aren't familiar with, I also have the page itself uploaded at two different locations.
To the right, on the second column (actually, row), it says:
Roughly translated, it says:
[Constant-release type] Zanpakuto that constantly take their "shikai" form due to their wielder's great spiritual power. Ichigo and Kenpachi's zanpakuto belong to this type.
So there you have it. It is officially stated by Tite Kubo himself: Zaraki Kenpachi does have shikai.