Install the Quicksilver beta from blacktree... once in quicksilver (you can make it a startup item with no dock icon too so that its completely in the background), go to preferences. If you can't get there, you invoke quicksilver by hitting control spacebar, then get to preferences by hitting command , (comma). Go to Triggers, hit the plus sign. Select "Hotkey." Start typing the app you want to launch. "Open" should be the default action, but if it isn't, hit tab to advance to that field, then type open. Click save. Then you click the empty space where it says "Trigger" (have), and go under the settings pane. Click into the "Enter hotkey" field that is flashing, and then hit the key combo you want to assign. You've got some other options that are fairly self explanatory, like if you want to launch it on release of the keys, on hold for x amount of time, or just on press.
For the record, I usually make my custom hotkey commands contain both command and shift just to prevent overlap with other App key combos.