We're kind of hoping your interest in Green Day spans the Dookie-era and, well, only the Dookie-era. Anyway, irrelevance. What's important is the teaser image at the end of the Green Day: Rock Band demo. It shows teaser images of guitars, bass, vocals, drums and... keyboards — followed by a big "3".
What could it mean? It means keyboards are going to be in Rock Band 3. We're almost 100% certain at this point that Rock Band 3's packaged keyboard will have light-up keys, which you'll have to follow in order to score points in the game. This would play into the rumors suggesting that Rock Band 3 will teach you a real instrument — hitting real keys should give budding players the feel of a real keyboard, with those "light-up" keys optional once you've learnt the piano-line. Naturally, these techniques would transpose to a real piano/keyboard essentially teaching music through video games.
If this doesn't happen then we're off to trademark the idea ourselves. It's obvious.