The first release of AppStream-Core is now available, which provides support for creating the AppStream database and accessing it via a GObject-based interface. This basically comes down to the simple creation of "Software Centers" for Linux.
The AppStream-Core 0.1 release was made today by Matthias Klumpp and its current API is deemed unstable and is intended as a "play around" release should you be interested in working on software centers/stores for Linux. AppStream-Core is a requirement for those wishing to use the Ubuntu Software Center on their preferred distribution. GNOME Software will also likely end up developing a dependency upon this new software package.
For more information on AppStream-Core 0.1, see the release announcement