I changed /bin/sh to dash when it was still called "ash". At once it reduced my booting time by more than a third of what it was.
That was probably 5(?) years ago, and the "trick" was then already well known to people reading debian-user. It became even more popular after Nokia did it as well in their first N700 tablet.
I think it bears testimony to Ubuntu's commitment to improving the user experience that they did this change. The fact that so far, AFAIK, they were the only dist that did it, shows IMO the lack of actual focus on desktop usability by most Linux distributions.