You and me both want to know; I've been hoping they'll create something at least a tidbit more friendly than having to use a hacked module together from the Freescale documentation on the subject, and that they'd provide us with a kernel module for it (some kind of interface we can just pick up and run with in our code), but it seems they've migrated their work to the 2.6 kernel, so we've gotta wait on that to make heads or tails out of it.

For now, the Freescale docs on the MX21 are pretty complete and there's an example kernel module written by bunnie which uses some rough-around-the-edges code that will get you started at the very least (which may or may not work with the 2.6 kernel, I'm not sure at all what changes may need to take place to make it compatible with future Chumby's).

For the Docs, head over to Freescale's page on the MX21, you can't possibly miss them. For the kernel module and some demo code: http://files.chumby.com/resources/Demo_code.zip . [Mainly, all that's relevant in the file are the spi_*() functions and their related variables/macros, and it's fairly well documented].

Hope that gets you started, if you need any help, feel free to contact me.