The current state regarding Linux distributions on the Apple TV is a bit of a mess, I will try to give an overview here.
Note that only the first generation of the Apple TV is known to run Linux. Later version run on Apple A series ARM SoCs with a PowerVR SGX (and later A10X Fusion). Because there aren’t any free drivers for this GPU, this isn’t very useful for a settopbox that has video decoding as its primary use case. The proprietary drivers have only been used on OMAP SoCs with Linux. It might work with Openiboot, such as this one.
The CPU is an second generation Intel Pentium M, codenamed Crofton. According to Apple Insider, they used an underclocked Dothan core to be able to run on a 350MHz bus. Although according to Wikipedia, the lowest clockspeed for Dothan CPUs is already 1GHz.
The GPU is an Nvidia GeForce Go 7300 with 64MiB of VRAM.
The OEM software is the Apple TV Software based on Mac OS X 10.4 Tiger.
People have used the Broadcom Crystal HD on a mini-PCI Express card for accelerating video playback. Because this mPCIe card is replacing the original Wi-Fi mPCIe card, it has been recommended to switch to USB Wi-Fi.
The kernel driver has a long history, both temporarily in /drivers/staging/ and floating around in various Git repositories.
The history of this kernel module has been documented here: https://github.com/dbason/crystalhd/blob/master/HISTORY.md