I wrote a plugin for decoding MP3's using the mpg123 library ( http://mpg123.de/ ).
It can be found on Github: https://github.com/dv1/gstmpg123

- Tested on x86-64, using Arch Linux, and on an ARM Cortex-A8 , using Angstrom
- Relies on mpegaudioparse for seeking and extracting the frames
- Uses GstAudioDecoder as base class
- Works with GStreamer 0.10.36 and 0.11.92
- Supports multiple downstream sample formats, making use of mpg123's internal code paths for each one

I know of of older plugins that decode with mpg123, but from what I have seen, they are no longer maintained, and were using GstElement as base class (I use GstAudioDecoder). I also heard about seeking problems.

Also, I am aware that GStreamer uses autotools (and not waf) for building, and am currently writing autotools scripts that should be compatible with the GStreamer ones.

It has worked well for me so far, I have watched movies with MP3 audio with it, didn't see seeking issues... the only feature left open is gapless playback, and the discussion about that one can be found here: https://bugzilla.gnome.org/show_bug.cgi?id=620323