I came across it when news about the 0.3.5 release was posted on Slashdot, and had a bit of a go at helping out. Here are some bits and pieces you might find useful.
Note that Kurt, the original developer, appears to have left the project. It has been forked, under the name Syllable. The information below was written in 2002, so I don't know how much of it is still correct. Use at your own risk!
AtheOS is lacking in info for programmers at the moment, so I've been writing down notes on what I've been looking through. Please take a look, and tell me if I've made any mistakes, or if you're able to fill in on details that I haven't picked up.
Along with various others on the atheos-developer list, I'm thinking of putting something together to make it much easier for both beginner and advanced users to install apps properly. Triumvirate wrote a program called AtheInstall to do the job for apps that need a graphical installer (à la InstallShield), which is a great start; what I'm doing is more like the automated installers (apt, dpkg, rpm) to be found in various Linux distributions.
This is the only way I can find to share files between AtheOS and any other OS. Works really well when coding remotely in UltraEdit on win32, which can read/write from ftp servers.