Not that I have read any of his books but I've watched the documentary and several of his magazine articles and he totally seems like a centrist. On some subjects (like Islam/immigration) maybe even slightly right of center.
Why is he considered a Marxist?
One reason why Žižek is considered Marxist is that his philosophical genealogy, coming through the Frankfurt School and Critical Theory, can be traced back to Marx and left Hegelianism, even though his thought has since departed from traditional Marxism.
Another example of this is Jürgen Habermas, who came out of the Frankfurt school and can trace his philosophical "ancestry" back to Marx, but whose current thought is definitely center-right (I recently saw a lecture by him advocating for a return to religion :-D ! ).
Cases like this I think are showing that the traditional left/right dichotomy no longer holds, if it ever held at all.
Here's a lecture by Žižek where his more traditional Marxist tendencies are a little bit more obvious.
