I'm surprised not more people are interested in this feature. Mobile data subscriptions are becoming expensive and all have strict download restrictions (well at least in Holland).
It would be great that when I'm connected through wifi (normally without download restrictions), my whole playlist would be downloaded and while connected through 3g, only a selected number of songs are preloaded.
Anyway, manually works for me too atm. I just must not forget to put it back or else my data bundle will be gone
