Well the game is apparently supposed to come out in June, or sometime this summer, so they would have to come up with an official list sooner or later.
I could see that both happening and then not happening at the same time. If their most popular songs were to not be put on the game, then the game would surely not be as successful. I mean, sure fans of Green Day may still get it, but only having American Idiot and some other popular songs and then songs probably not a lot of people listen to is just wrong.
At the same time, if all of their truly awesome songs were to be released only via downloadable content, they would have been so already. Green Day has been requested on Rock Band ever since it came out, and so far we've only gotten two packs from 21st Century Breakdown. Something tells me that the producers were getting it in their minds that because of all the requests for Green Day, the best thing to do would be to make a band specific game.