I'd suggest MenuTemperature but I think it uses SpeedIt as well, which might be the problem. SpeedIt was only released for 10.4 and they only mentioned a 10.5 release being delayed way back when the GM copies first shipped for Leopard, with nothing added after the 10.4.10 release on InCrew's site.
The only other way I know to read system temps is with CoolBook, which reads system temp, speed, and voltage; registered it also controls voltage and clock settings. You might have some luck as the developer released updates right up to 10.5.2 so it's at least been maintained.