I saw this from a link from nixCraft in the week and meant to put this up. It has a couple of great examples of how to find out the temperatures of your Raspberry Pi, this does both the CPU and the GPU.
It has examples of the commands, plus it gives a good example of how to put the commands into a simple shell script so you can change the way the information is displayed.
I am always interested in feedback so please feel free to add any comments, or you can mail me here. If you would like to submit a quick tip with full credit and links back to your site then also feel free to contact me.