On the project's homepage, two reasons are given for why the project is going into hibernation:
1) Scientifically, we're at the point of diminishing returns; basically, we've analyzed all the data we need for now.Via ARS Technica
2) It's a lot of work for us to manage the distributed processing of data. We need to focus on completing the back-end analysis of the results we already have, and writing this up in a scientific journal paper.
However, SETI@home is not disappearing. The web site and the message boards will continue to operate. We hope that other UC Berkeley astronomers will find uses for the huge computing capabilities of SETI@home for SETI or related areas like cosmology and pulsar research. If this happens, SETI@home will start distributing work again. We'll keep you posted about this.