Cosmoe, Bill Hayden's fork of AtheOS, seems to be progressing nicely, and it looks like he's trying to integrate almost everything under one system.
It runs on the Linux kernel, and he has a wrapper for the Carbon API and is modifying the AtheOS API with the goal of source-code compatibility for the BeOS API.
"Cosmoe strives to be source-code compatible with BeOS and MacOS (Carbon) code. Windows source-compatibility is being explored." [Cosmoe FAQ]
The impact of this is that it may soon be possible to recompile both BeOS and MacOS applications to run on Linux - a great opportunity to allow Mac developers to write more cross-platform software.