There are a bunch of different Linux kernel options for MIPS computers. This page has links to the various possibilities.
- Standard linux kernel. As of 2.6.18.2, this doesn't include any architecture support for Broadcom 4780 SOCs. It has MIPS support for other architectures though.
- Linux-MIPS kernel. The Linux-MIPS project has a GIT repository where they include a bunch of additional MIPS-related stuff into the vanilla Linus kernel. Still nothing for 32-bit Broadcom chips like BCM4780, which is too bad since they track the Linus kernel very closely.
- OpenWRT project. In their subversion repository, "kamikaze" branch, there are directories brcm-2.4 and brcm-2.6 under target/linux. This adds BCM4780 architecture support. They update these patch sets periodically to target more-recent kernels.
- The standard Asus firmware comes with Linux 2.4.20, probably with modifications. This source is part of the GPL firmware.
It seems as though the OpenWRT kernel is by far the most viable choice. Once we determine the minimal patch-set necessary for FreeSA, we'll start trying to keep it up-to-date against new kernel versions ourselves.