Green Day: Rock Band demo from Xbox Live Marketplace today (gold subscribers only) found a little bonus waiting for them when they exited the demo: The teaser image posted above. Alongside the series' four iconic instrument logos is a whole new symbol, which seems to indicate that keyboards (or perhaps ... keytars?) will be added to Rock Band 3.
You can check out the demo, which includes the songs "Boulevard of Broken Dreams" and "Welcome to Paradise," to see the teaser firsthand, and to start imagining all the wonderful implications a Rock Band keyboard might have.
Xbox.com: Add Green Day: Rock Band demo to your Xbox 360 download queue