Select in function?

I have the following function:

CREATE FUNCTION RegistrarDesconexion( integer )
RETURNS integer
AS '
BEGIN
UPDATE BitacoraConexion
SET Desconexion = CURRENT_TIMESTAMP
WHERE IdBitacoraConexion = $1;
SELECT Desconexion FROM BitacoraConexion
WHERE IdBitacoraConexion = $1;
IF FOUND THEN
RETURN 1;
ELSE
RETURN 0;
END IF;
END;'
LANGUAGE 'plpgsql';

and if I run:
select registrardesconexion (5);

I get the following error:
ERROR: unexpected SELECT query in exec_stmt_execsql()

I don't know what's wrong, though I assume it's saying I'm not allowed
to use the select statement where it is.

Any ideas or comments.

Thanx.