D20 Ars Magica
The D20 System was a very popular rules system for role-playing games from the years 2000 through 2008. In fact, it remains popular to this day but for legal reasons no one calls it the D20 system any more (they call it OGL instead).
D20/ OGL Combatibility
Ars Magica is not based on the D20 System and is not inherently compatible with it. There is no official D20 version of Ars Magica as such, and it would not be a simple task to convert one game system to the other. They're different in a lot of ways, especially in how characters advance in ability and how combat works.
Black Monks of Glastonbury
The Black Monks of Glastonbury, by David Chart, is part of Atlas's Coriolis line, and is designed to be cross-compatible with both Ars Magica and the d20 System. The adventure is not an attempt to convert Ars Magica to d20, but simply an Ars Magica supplement with notes on using it with standard d20. In the words of the author, "Black Monks is a dual-use supplement, not a conversion." Atlas apparently has no plans to do a d20 translation of Ars Magica.
Fan Projects
There are unofficial fan projects to convert Ars Magica to D20
- http://www.users.qwest.net/~jordanerik/Gaming/ArM/ Please note that this is a conversion for ArM4.