Everyone sees to have missed the fact people would have to be willing to recreate songs for the DS, and without any software for this at the moment, it would be quite hard. All the custom songs at the moment (and the tools used to create them) use 5 notes, rendering them incompatible (I assume, unless you can just miss out the 5th fret when playing).
Anyway, it would be a good opportunity. Though I'm looking at GHIV Wii really. VV confirmed DLC will be downloaded onto your SD card, which means songs will be read off there too. Anyone with a brain will realise what opportunities this creates.