As Joe mentioned, Dire Spirits is now available. It's an introductory scenario to show people Nyambe -- you don't need to own the Nyambe book to play it. (And if you need more than the 4 pregen characters in the book, there are two more on our website for download.)
We do plan to do some more supplements for Nyambe. One that is in the works is a book of Nyamban magic items. We think this is a good approach because such a book would be useful to any d20 player, not just people who already own or play in Nyambe. That should give it the widest possible audience. ("Nyamban" magic items could show up as treasures in any campaign world.)
Also, a Nyambe-Arcanis crossover scenario makes its debut soon in a Living Arcanis event at Winter Fantasy, thanks to the good folks over at Paradigm Concepts. Should be cool!