We should require Java 6 for release 3.1.0. At this point there is full java6 support on all major platforms (linux/windows/macosx).
The vast majority of ZooKeeper contributors use Java 6 when developing, and hence do run into (usually minor) back-compatibility problems.
I believe there are also some benefits regarding JMX (updated spec support in java6) if we drop java 5.
FWIW: Hadoop Core has also moved to require Java6: