Past time I have been spending on developing a DMX Library for Arduino which is easy to use and understand. It is a UART (Serial) based library that can be used similarly as most other components on the Arduino platform.
The library can be downloaded from: http://sourceforge.net/projects/dmxlibraryforar/.
There is also a wiki available at the same link which will show you the basics of how to use it and else you can simply open one of the examples shipped with the library.
Currently I am also busy with implementing RDM (Remote Device Management) capabilities which will I will make available for download soon.
The first version will only support the minimum functionality as required for DMX devices according to the RDM specification. After releasing that I will plan which features will be added first and after that. If you prefer certain features above others you can let me know and I will take that advice into consideration.