Bracketing. Why you can't do it with self timer mode? Why must we press shutter release button for each »bracketing« separately? If anywhere, than with bracketing we do not need shaked images. They must match perfectly!
I don't know if Nikon engineers wanted to, but they fixed this problem. Now you can have an "auto bracketing" in self timer mode. In menu C3 choose self timer and in same menu select the number of shoots you want to take with self timer. This feature (number of shoots) does the trick. Set it to 3 shoots and (to make bracketing easier) set self timer to 2 seconds. "Engage" bracketing. Press shutter relese button and Nikon D90 takes 3 shoots automaticaly. Each exposed differently, as you set it in bracketing option. Nice, ha?