If you're a purist, you probably believe that one can only become world champion by defeating the title holder in a head-to-head match consisting of of many (12-24 or more) games. That's how it was done from 1886-1993, and I think most chess players (myself included) prefer that format. But Topalov is technically recognized by FIDE as a World Champion.

More here if anybody cares: http://en.wikipedia.org/wiki/World_Chess_Championship