AusGamers has a video interview (and transcript) up, chatting to id Software's Matt Hooper about all things RAGE.
From the interview:
"There are specific blueprints and then upgrades to those blueprints. But again, we didn’t want 10,000 or one million different weapons and items - we wanted a substantial list and we probably tried a million different things, but we pick and choose the ones we think make the difference and then we know the player will use those. So it really is a shorter list, it’s not as dynamic as you might first expect..."