http://www.research.ibm.com/journal/rd/434/westerink.html

Well worth a read.

Excerpt:

"To assign bit rates to specific segments of a video sequence such that we attain constant visual quality, it is optimal to have knowledge of the characteristics of the entire video. This can only be done by playing the whole sequence and gathering certain statistics over time. Thus, a VBR system for DVD, in which we wish to distribute the available bits optimally over the video, is essentially a multiple-pass system. A one-pass VBR system can be designed as well, but it will always be suboptimal."