Good question, based upon what I have experienced myself and observed others there is enough variation to prevent adaptation and see steady strength and performance gains.
That said I think it is hard to have to much variety and I am hard at work developing a randomized workout generator that people who have a solid base (above level 5) can integrate into the existing program. The random workout generator will take the baseline of push ups, pull ups, and squats and create a random workout based on the athletes ability, using a larger variety of BW exercises.
Starting strength is an awesome book and I am a huge fan. I would also be interesting in seeing the protocols applied to some of the BW exercises listed here maybe you could be a lab rat. http://simplefit.org/phpbb/viewtopic.php?t=183
I really like body weight exercises because it is hard to beat them for simplicity, convenience, and economy which is the mission of this site.
All roads lead to crossfit :) when I created this site it was my goal to make the crossfit protocol accessible to those who may not have the time resources or conditioning to jump right in to the wod, however I will continue to make this site as complete a program as possible with BW exercises packaged in the most simple accessible format possible.
Great to have you here and thanks for the good questions, and appreciate all feedback.