Matrox just sent this press release this morning regarding this new game called Warmonkey that's supposed to look and work on G400's dual head like a charm:

As the first true 3D action/tactics game, WARMONKEYS features breathtaking graphics, revolutionary artificial intelligence and gameplay that is both challenging and addictive...

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Silicon Dreams is currently working in partnership with Matrox Graphics Inc., whose Matrox G400-based graphics cards feature DualHead Display—offering users the ability to simultaneously output independent images to two display screens. When playing WARMONKEYS with a Matrox Millennium G400 Series card, gamers will be able to display the 3D view of the world on their primary monitor while using a second monitor or TV to display other views such as maps and first-person views of objects being shot at

Here are some pictures of the dual head in action, as well as a Matrox coffee mug (I don't know either): [shot 1] [shot 2] [shot 3]