I got along a bit further in the FAQ and found this:

In reply to:

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The player's Linux kernel is heavily modified from the original Linux kernel, and this modified source code is made available.

The Linux Emptool utility is open-source.

The tool which uploads data to the player's flash RAM (for things such as custom kernels or logo images) is open-source.

The specifications for the hardware and all input/output connections are freely available.

The actual player software and the Emplode utility are proprietary, and their source code is not made available to the public. However, there is enough available information for someone to develop their own player software if they were willing to expend the time and energy required.

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The player software (or something very similar to it) is being used in the new Rio Advanced Digital Audio Center http://www.sonicblue.com/audio/rio/rio_audiocenter.asp