Here's the announcement:
Water is coming to LittleBigPlanet as part of an exciting new kit!
It's not just a saying you know - We really do need to test the water to make sure the new features all work as planned, and we need your help to do it! How would you like to dive on in to the crystal clear waters and enjoy some splish-splashing about before everyone else? Get out your rubber rings people, this is gonna be one hell of a pool party!
If you're interested, you just need to be 18, have a PSN ID, and promise not to ever never never ever never share what you see with those who write about video games. Have fun!
LittleBigPlanet Beta Sign-Up [site, thanks Bob-a]