We use r23044 because shortly after is when the 2.4 kernel went away.

$ svn co -r 23044 svn://svn.openwrt.org/openwrt/branches/backfire
$ cd backfire
$ wget http://wrt.scottn.us/openwrt-backfire-adm8668-r1.tar.gz
$ tar vfxz openwrt-backfire-adm8668-r1.tar.gz
$ rm target/linux/generic-2.4/patches/100-wireless-extension.patch 
$ cp toolchain/uClibc/config-0.9.30.1/mipsel.brcm-2.4 toolchain/uClibc/config-0.9.30.1/mipsel.adm8668
$ make menuconfig

Here you select 'adm8668' and that should be all that's required.

$ make

Take a nap.