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.