There is no way that I know of to do it from the OS, but there are programs that will allow you to do it. QuickSilver will allow you to do exactly what you want using triggers. (I am such a prostitute. EDIT: I used a different word but evidently it is considered swearing by the filter. It rhymes with war. ) I use <CTRL><CMD>c to launch Camino as an example. <CTRL><CMD>v to launch voodoopad. Various combinations using the arrow keys to control iTunes. So on and so forth.
There are other programs that will allow you to do it though I can not come up with them off the top of my head. Launchbar and/or Butler can do it I think. A little searching on MacUpdate brought up Global HotKey, FunKeys , Gestures , and iKey . You could try one of those.