It depends.  If you yourself are offering the job, and if you only want to
offer jobs at fairly intervals, then I don't think many people mind you
advertising them here provided that:
   a)  You make it clear in the post's subject line that its a job.
   b)  You make it clear in the post's subject line /where/ the job is.
        (And remember that there are only a few cities in the world where
       just the name of the city is enough information.)
   c)  You don't repeat your posts.
   d)  You monitor the newgroup and /reply/ to questions and respond
         to criticism (or learn from it).

If you are an employment agency, or if you are unwilling to behave sensibly in
a public discussion forum[*] (as above), then the rules are very simple indeed.
You are not allowed to post job adverts here.

   -- chris

([*] I guess you /are/ willing or you wouldn't have asked this question in the
first place ;-)