Their FAQ on the support site has been updated. Pretty confirmed:
Originally Posted by EA:
I've heard ROCK BAND will not be available after May 31. Is this true?
Yes, we will be suspending support of ROCK BAND after May 31 and focusing resources on other EA titles. We thank everyone for playing ROCK BAND, and we encourage you to explore some of the other exciting titles in our mobile line-up.
Can I still play ROCK BAND if I've already downloaded it?
The ROCK BAND servers will be live through May 31, 2012. If you have already downloaded the game, you can continue to play until then.