WDTV, Sigma Designs and GPL source code for Linux

I've been looking for a long time for a legitimate download site for the sources of Sigma Designs' modified Linux kernel for SMP8634 chips.

I did find one very dubious-looking FTP URL in a forum, supposedly from a device manufacturer in Korea, but that leads to a lot of kernel source files which say "proprietary and confidential" on them and no indication that they are distributed under the GPL.

Western Digital's WDTV support site does provide the modified kernel source, which is a refreshing change!, although the copyright notices on many individual kernel files seem inconsistent with the GPL. Still, looks like GPL compliance is slowly improving with these Sigma Designs vendors, and Western Digital are to be commended for being better than average.