I don't know if you are familiar with the embedded world, but a large percentage of the offerings (specifically system on chip or SoC) have NDA requirements for the documentation.

And many of these platforms don't have a decent Linux port, even though they could support it.

Even with SoC vendors such as TI, the graphics core for the OMAP4 (for example) is a binary blob. There is no documentation for that, and no hope that situation to change (in part because TI has exited the mobile phone market).

Please tell us of your mobile platform experience, and what vendors you consider open or closed.