mp3taglib is a id3v1/id3v2 mp3 tagging library for ruby, based on id3lib. At this early stage it already supports the retrieving and setting of most of the fields of id3v2 tags. Mp3 encoding informations like bitrate can also be retrieved.
You can download the last release from RubyForge. You will also need to install id3lib, including the header files (devel packages).
To install it, simply type as root (as least for the 'make install' part) : ruby extconf.rb && make && make install
A file example.rb is included as an example of how to use the library.