To expand on Thrommel's original post and to address concerns about OGL vs D20 that some people migh have, the following questions and answers below are right from the AU FAQ on Monte's site.
Q: Is this a d20 product?
A: No. It's been created under the Open Gaming License, but not under the d20 License. Basically, the main difference is that under the OGL, we can provide character creation rules.
Q: Is it a stand-alone game?
A: Yes, actually, although it utilizes core rules familiar to most players, and it can also be used as a supplement for existing Open Game Content games.
Q: Is this just another book of classes, spells, and feats?
A: No. It is quite literally a variant player's handbook. You can use this instead of the player's handbook you have. All the races and classes and rules in this book have been designed to form a cohesive whole. They are balanced to work together and created to be complete in this book. That is to say, there are no traditional "roles" you'd like a fantasy game to fill that this book doesn't cover -- but it covers them in a way that is unique. People who want to play spellcasters can choose to be runethanes, who draw their magical power from symbols and sigils, or a witch who not only casts witchcraft spells but manifests one of many types of specific and mysterious witchery powers. Those who favor the more heroic warrior can play an unfettered, who uses wild tactics as well as grace and speed in combat, or a totem warrior, who calls upon the power of a particular animal spirit to grant him special -- often savage -- powers. And this is only the start.
Q: I already have a player's handbook. Won't I be purchasing a lot of the same material twice?
A: Not really. Every class in Arcana Unearthed is completely new. Every race (except humans) is completely new. Magic works differently. Most of the feats are different. There are even some interesting changes to the skills and the combat system. You don't have to utilize all the changes if you don't want to (the classes, feats, spells, and so on will work with the traditional rules), but the material offers you some interesting options. Besides, if, as a player, you choose to use a race and class from the Malhavoc Handbook, you'll then have everything you need in one book rather than two.
Me again. In short, if you want to publish some stuff (like character generation rules) you must use the OGL. It is not a repudiation of the D20 license at all. Just a different set of rules governing what can be published.
And don't be apprehensive about AU. While it is published under the OGL, it is compatible with core 3.0 and 3.5 without hardship or trying to turn the system on it ear. During our group's playtest, we used a mixture of core 3.0 classes and AU classes without problems. The AU spell system can be adapted to a 3.0 campaign without much trouble (in fact, our group is using the AU magic system in our standard 3.0 campaign - we like it that much!)
Hope this answers any questions that people may have about OGL and 3.0/3.5 compatibility.