Booting code through the notes exploit has proven to be too uncomfortable in the long term, as you break the apple firmware that way, but still have its non-neglegible bootup times. The Rockbox bootloader is faster, but still too slow. This is why iLoader has been developed. iLoader replaces the whole firmware starting from the second level bootloader, and thus gets booted up directly by the bootrom. It then shows a boot menu and allows you to boot different firmware images, which can be stored on the data partition to allow easy updates. The boot menu of iLoader is fully configurable.
iLoader only works on the 2G Nano, as this is the only iPod we've figured out the FTL for.
For installation instructions, see the iLoader homepage.