I use SpeedFan which can monitor motherboard temps and fan RPM's as well as S.M.A.R.T. info from your hard drives. It works perfectly with my ASUS motherboard, but I haven't tried it on the intel-based Macs.
I'd like to know this too. I tried speedfan and al it monitors is the HDD temp. No CPU. Tried a couple others too and diito, no CPU ability to monitor temps. I'm beginning to think it can't be done, yet anyways.