Here are the PCB sources for the latest revision: http://elinux.org/Beagleboard:BeagleBoneBlack#Hardware_Files
The datasheet for the AM3558 processor is here, no NDA required: http://www.ti.com/product/AM3358/technicaldocuments
The bootloader is U-boot, which is open source, based here: http://www.denx.de/wiki/U-Boot
The GPU in the AM3558 is a PowerVR SGX530, which is not open source in any sense-- I believe that the Linux driver uses a binary blob (but I'm not 100% sure of that).
For comparison, the Raspberry Pi has open schematics (http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspbe...) but not open PCB layout source files. There's a partial datasheet for the processor here: http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM283..., but you can't buy the processor from Broadcom unless you're willing to buy 100,000 or so.