From the FAQ question (which is the reference to follow, I only copy the current version here, it might not be actual in the coming months):
How does a post become a Community Wiki post?
There are several ways a question or
answer can enter community wiki mode,
and most of these ways will occur
automatically based on the rules of
Posts enter community wiki mode when:
- The body of the post has been edited six (6) times by at least four
(4) different people.
- The post has been edited eight (8) times by the original owner.
- The post's author checks the community wiki checkbox when composing the question or answer.
Note that this checkbox is
only available to users with a
reputation of 15 or more.
- The post is edited by its original author, who when doing so opts to
check the community wiki.
- An answer posted to a Community Wiki question will also be Community Wiki.
Note that when a question is
made CW after answers have already
been posted, the existing answers are
not normally converted to CW
- The question generates more than 30 answers (15 on Super User). In this case, the question and all
answers will enter community mode, as
will any future answers.
- A moderator has reason to believe that the question serves better in
community wiki mode.
* note that the rules outlined
above are based on observations; the
actual behavior of SO is subject to
change without notice, and may well
have done so since this was last