Where are downloaded songs going to be saved on the PS3? The obvious place would be to put them in the saved game files. The problem with that is how accessible game saves are this generation. I can't think of a way that Rock Band could get songs on your console without leaving the door open to custom songs. I can't think of a way that prevents someone from making an song/note editor that makes RockBand save files. Time and time again I've seen encryption schemes thwarted so I don't think that would be a deterrent for very long.
I can't even think of a way that keeps people from sharing official downloadable songs short of forcing you to be signed in to play. If thats the case it would be pretty lame if the internet is out and you can't play 20 songs you paid good money for.