Both Keith Robinson and Merlin Mann discuss how creativity fits into the GTD system.
As Keith shows, you can use GTD to setup a framework for doing creative work. You can write lists of things you want to be creative about. You can use GTD to process the outputs of your creative thinking. If your brainstorming yields five ideas, you can put down "investigate idea #1, investigate idea #2, etc" on your to-do list. You can use your GTD system to capture all your ideas. But the one thing that GTD can't do is make creativity like cranking widgets. I suppose you can make a to-do item like "Think about X for one hour". But then the next action (assuming that you don't come up with any ideas) is to "Think about X for another hour."
GTD is a system for processing, not coming up with ideas or brainstorming. One should recognize that limitation and remember to keep up to date with the @someday list!