Hello!
I am the main developer of the ArmyCalc project:
http://armycalc.com/
I am curious if someone would be interested in contributing to the project and create rulesets(data files) for WH40K.
Project consists of open-source online calculator and downloadable open-source desktop application.
This tools are and will be free.
I am also working on a mobile version (for Google Android, maybe iPhone someday) that will probably be a commercial product.
All those tree parts of the project are using common data file format.
Rulesets file format is based on XML. Lua scripts can be used for advanced rules checking or custom displays.
It is described in details here:
http://armycalc.com/view_doc.php?doc=twr
All currently available rulesets can be viewed for examples.
I'll be happy to hear any opinions and answer any questions.