Mounting iPods on Linux is harder than it should be. OS X formats them by default as HFS+ with journalling enabled, and Ubuntu can't handle the journalling, so you have to turn that off in OS X by running
sudo diskutil disableJournal /Volumes/iPod
Even then, if you remove the iPod without unmounting it first, the filesystem gets marked as dirty: "hfs: Filesystem was not cleanly unmounted, running fsck.hfsplus is recommended. mounting read-only.". You can get fsck.hfsplus from Darwin, patch it and compile it, but it just gives a segfault on my machine.
The final thing that turns up when searching for a solution is hpmount. After trashing one external hard drive (some of the files were recoverable with a utility on OS X) and just trying it again on an iPod whose filesystem also got trashed (at least there was nothing irreplaceable on there), I can definitely say don't use hpmount.
If you get the error "mount: wrong fs type, bad option, bad superblock on /dev/sdd3, missing codepage or other error" after using hpmount, bad luck.