GHI Electronics has announced the Fez Hydra Mainboard and kit. 

The Fez Hydra is based on a 240MHz ARM9 processor, and its hardware, firmware and software will be fully open source. 

Fez Hydra pre-orders will begin in December.  The mainboard will be available for $79 alone or with several kit modules for $100.  Learn more at http://www.ghielectronics.com/catalog/product/332.