Some insights about the development of Atlas behaviors
Nikhil has an excellent post about developing a client-side behavior with Atlas. If you're thinking about developing your own Atlas components, this is a must-read:
http://www.nikhilk.net/Entry.aspx?id=107
UPDATE: There's also a more entry-level, page developer oriented video on Atlas from Nikhil on MSDN TV:
http://msdn.microsoft.com/msdntv/episode.aspx?xml=episodes/en/20060119AtlasNK/manifest.xml