Last week a vulnerability for iPod Nano 5G was discovered, and there is an exploit already, which jailbreaks the device. it cannon be fixed even if Apple still provided updates, as the buggy code is read-only (in boot ROM area).
https://github.com/freemyipod/wInd3x
There is a lot of work until something usable (like Rockbox) can be seen, but anyone who would like to help with the development is welcome IIUC.