Atari is back with a website covering technical details on coding for Dreamcast using the FENIX/DIV engine:

It's compiled from various sources, and I'll be adding more info as I find it. I've also put up a log of the development of my game Xaz on the site. Visit http://www.briandeuel.com and click the 'game console works' link to view these pages.

If you're interested in porting or developing a game using the FENIX engine, check out his tutorials - They're very easy to follow.

Source: DCEmu UK Dreamcast Dev Forum