Regardless of whether you've spent money on DLC or not, regardless of how much you've played and enjoyed the app, EA have announced that on May 31st Rock Band iOS will become entirely unplayable. Why? How? EA are yet to formally explain the move, but considering that people have paid for access and are now having that access take away, there's not a huge amount to explain.
Despite the complaints coming from the Rock Band forum, this has little to do with Harmonix, who have little to do with the app. HMXHenry hazarded a guess as to what the problem may be:
"No clue as to the specifics, but my guess is that it has something to do with the MTV Games and EA logos that pop up when you load it. Since we are no longer partnered with them, I have no idea how that has affected projects like RB on iOS. But, as we've seen with the recent DLC delays, there's a lot of work being done behind the scenes now that Harmonix is independent and I wouldn't be surprised if this was related.
"Again, that's mostly guesswork, but I'm not at all familiar with the particulars of the partnership or how content is managed on iOS."
We'll update as they find out exactly what's going on.