There are two ways you can achieve this:
* Use applications like Launchy or Gnome-Do that make it easier to launch applications by typing a few characters of the application’s name.
* Or you can summon gconf editor (Alt+F2 then type gconf-editor and hit enter), and navigate to apps > metacity > global_keybindings, double click on any of the run_command_N and type in the keyboard shortcut you want to assign to an application then make a mental note of the number N. Then go to apps > metacity > keybinding_commands and double click on command_N (N being the number you used above) and type in the command you want to run. As an example if you wanted to run Firefox you would type in firefox.