Released shortly after Upstart 1.8 is systemd 199. Features of systemd 199 include systemd-python gaining an API to expose the libsystemd daemon, SMACK setup logic improvements, behavior changes to several components, a few kernel sysctl variables being set by default, new tunables, a new libsystemd-bus module that implements a near-complete D-Bus client library, journald improvements, and much more.
The libsystemd-bus module is a minimalist yet complete D-Bus client library that doesn't aim to be portable, abstract, and tries to be nice in C. This systemd implementation is much leaner than upstream libdbus. Meanwhile, kdbus is an ongoing systemd project to provide a new kernel implementation of D-Bus. The libsystemd-bus implementation is a first step towards this Linux kernel D-Bus. The plain is to stabilize libsystemd-bus, porting over code to use this new module, and to then switch over the system bus to using kdbus. Details on those systemd/kernel D-Bus plans are shared in this mailing list post.
More details on the systemd 199 release can be found via the mailing list release announcement.