According to Computerworld, Apple has shipped a Java security update for Leopard and Snow Leopard that patches 27 vulnerabilities in the Oracle software. Some of the bugs could allow hackers to hijack a vulnerable Mac. As the article explains:
Mac OS X 10.5, a.k.a. Leopard, received an update that patched 16 vulnerabilities in Java SE 6 and another 11 in Java SE 5. The update for the newer Mac OS X 10.6, a.k.a. Snow Leopard, also patched the 16 bugs in Java SE 6.