Six kernel vulnerabilities have been published which affect Mac OSX including 5 which can be used to exploit 10.5.6. They can be view on Milw0rm.com, see the link to the Apple specific exploits on the sidebar. These exploits can also affect Solaris kernels and FreeBSD. FreeBSD has been patched, Mac OSX has not as of yet.
Issue one exploits a remote heap overflow in AppleTalk network stack. The second and third exploits a memory leak which can cause the kernel to run out of memory. The fourth exploit relates to HFS vfs sysctl flaw which allows for a global variable to be altered without locking the mutual exclusion object (mutex). Mutex is used to allow multiple program threads to share resources, this is done by locking the mutex from other threads. In this case the locking process does not happen causing a potential of memory corruption.
The last has been know for some time, it relates to the HFS I/O control (IOCTL) handler. User supplied code can be inserted and executed with kernel level privileges.