The Audible User’s Guide for iPod is good to know about and includes this useful item:
Many Audible files are broken up into segments. The segments may be chapters, timed intervals, radio show segments, etc.
To use the section navigation feature, tap the center button once. Section indicators (when available) are indicated by vertical bars in the file navigation bar. Use the RW or FF buttons while in file navigation mode to jump from one section to the next.