Most often max certified crz level is not an absolute level as someone above stated. It's often a certified altitude of a 2 % buffet margin or sometimes higher depending on the aircraft. Also governing the max crz alt is the maximum cabin pressure differential. But there is always a safety margin built into that so the aircraft most often can climb all the way up to an absolute altitude...
so always follow TCAS what ever it might say.
As a note, you should be aware of the new communication prescribed in case of a TCAS RA issued by ICAO in May 2008.
Safe flying!