Open source status
The linux distributions running on RPi are claimed to be entirely open source, without proprietary binary blobs. (Broadcom GPU-interface blob was open-sourced October 2012)
The electrical schematics are available in pdf form here but the design files (reputed to be in Mentor Graphics Expedition format), BOM, and Gerbers are not available from the Raspberry Pi Foundation. A certain amount of reverse-engineering has been done (e.g. http://elinux.org/RPi_Partial_BOM_Rev2.0_ModelB).
The schematic bears a copyright notice but no other "proprietary" verbiage restricting either "clone" or modified implementations.
A Raspberry Pi Foundation representative  has spoken of their "intention to open-source the hardware" but it hasn't happened.