I have iPodLinux installed on my 5th generation iPod. It's interesting but not very useful for me. I normally run Rockbox, and I've only used iPodLinux for running Smartmontools smartctl to check hard disk health. Nevertheless, I'm interested in learning more about the Linux kernel, so I just spent some time tracking down the kernel source code.
Source locationsThe old home page at www.ipodlinux.org has been down for a long time. Don't bother trying Archive.org, because the site wasn't archived due to its robots.txt.
There is an ipodlinux SourceForge project. Its files section contains patches and binaries. The latest 2.4.24-ipod2 version there is old and does not contain PP502x code needed for 5th generation iPods. If you want to use it, get linux-2.4.24, apply uClinux-2.4.24-uc0.diff and then apply the ipod2 patch. You can build with the arm-elf-tools-20030314.sh toolchain.
The same SourceForge project also has a SVN repository which has a later version. The kernel was initially at trunk/linux/ and then in r2251 "The Great Befuddlement" moved it to linux/2.4/. The repository only contains changed files, so you have to take an unmodified Linux directory, apply the uClinux patch, and then copy files from the repository into that tree. This kernel builds with arm-elf-tools-20030314.sh and works with my iPod. The repository seems abandoned, with the last commit in 2009. For convenience I'm providing a Git repository of just the kernel.
The SVN repository also has a 2.6 port at linux/2.6/ but it's at a very early stage. It might partially work with some older iPod, but it certainly won't have full functionality. I didn't even get it to compile successfully yet.
The uClinux-2.4.x patches starting with uClinux-2.4.24-uc1.diff.gz also contain iPodLinux code. It seems to be a stripped-down version of the ipod0 code, meaning it's very old. This complicates attempts to upgrade later iPodLinux to a later kernel version version, but it wasn't too hard to do with a git merge.
The latest version?Finally I found ipodlinux on GitHub. The linux-2.4.32-ipod kernel seems to contain everything from the SourceForge SVN repository, but it switches over to uClinux 2.4.32 earlier, so it is not an exact copy. After that it contains some iPod specific patches, and then various Linux fixes.
The repository contains the entire kernel, so you can simply build from there. It is meant to be compiled with a later version of the toolchain. I used arm-uclinux-elf-tools.tar.bz2 with GCC 3.4.3. It compiles without problems and works on my 5th generation iPod.
Installing the kernel
You need to convert the kernel to a binary linux.bin image. Take a look at the instructions on SourceForge. Since I mainly use Rockbox, I have the Rockbox bootloader installed. It loads /linux.bin from the FAT32 partition if I hold down the play/pause key at the bottom of the wheel. You need a separate Linux partition for iPodLinux. If you need to resize partitions, be careful, because the original firmware partition is marked as unused but is actually required.