Finally sessioninstaller has left the status of vapoware! Sessioninstaller provides the missing link to use the PackageKit session API with aptdaemon.
PackageKit targets two use cases: a common package manager which could be used on several distributions and a set of common and very simple functions which can be called by third party applications to install e.g. extensions or codecs.
Therefor PackageKit provides two programming interfaces: a powerfull interface on the system D-Bus and a simple one on the session D-Bus. The first one is used by full blown package managers and the later one by third party applications.
Sessioninstaller implements the session interface of PackageKit, but using aptdaemon or alternatively synaptic as package manager instead of the PackageKit daemon. Aptdaemon provides a system D-Bus interface which is similiar to the one of PackageKit but provides a lot of Debian/Ubuntu specific features and is not compatible with PackageKit. Aptdaemon is used by e.g. software-center which is way ahead of any PackageKit based package managers.
This screencast shows totem installing a missing codec:
Corrsponding SessionInstaller and AptDaemon release information.