I found some information: Foveon Inc., and their only known licensee Sigma disclosed the XF3 RAW file format with documentation and some sample images. They are just free for pickup in PDF.

I haven't read it yet, but from a company that try to innovate in the area of sensors, I find that from Foveon it is a really smart move to disclose the file format. That means they don't fear that it will be used to compete with them.

Kudos to Foveon Inc.