For small events or club events, the player/arbiter is quite common, and normally it isn't a problem. For FIDE rated events it is discouraged, but not totally out of the question. However, if you wish to earn an arbiters norm (FA/IA) from a FIDE rated event, then you either have to be a non-playing arbiter, or forgo the norm.
In response to a question from Peter Cassettaru, the Chairman of the FIDE Arbiters Commission stated:
Dear Mr. Cassettari,
it is not allowed the Arbiter of a FIDE rated tournament to be a player in the same tournament at the same time.
Such an Arbiter's norm shall not be recognized.
With best regards
FIDE Arbiters' Commission