The (still incomplete) Fortinet source code has finally arrived. For those of you who're curious, I've made it available at ftp.gpl-devices.org. I'm planning to publish all "GPL code releases" by various vendors on that ftp site in the close future. This way you can avoid the hassle (and cost) to order a physical media via snail-mail.
The Fortinet Linux kernel seems quite a bit modified, especially looking at the network stack. No time to comment on that right now. If you're interested, RTFS :)