I don't have a dual core yet, but I do have some dual processor systems (same basic theory in CPU utilization). Basically the only way you will ever get both of your cores going at 100% is to have two threads (or more) running that use 100% of your CPU cycles. for instance, running two instances of Prime 95 simultaneously will bring them up. Im my case when I am rendering a 3D image or animation in Lightwave I tell the program to use two or four threads for rendering on the dual processor boxes, and it iwll use 100% of both CPU's for the polygon rendering...
CPU utilization is dependant on the software you are running.