The chapter support page at ID3.org was added in 2005. Support for id3v2.4 was added in 2000, yet that has had little support in players or media libraries - and even those that do seem to have some issues with consistency. I guess it is a bit of a chicken/egg thing which will drive more support.
There has been chapter support added for mp3tag in video files. Perhaps this will translate into audio mp3 files as well sooner or later?