Recently in Germany there was a cross-distribution meeting among the major vendors (Red Hat, Canonical, Novell, Debian, Mandriva, etc) to discuss a common application installer for Linux and one unified application store / market-place. The goal would be to have a common user-interface for application installation, how/what meta-data to use, determine a defined protocol for non-static meta-data, and decide what meta-data to share across distributions. Fortunately, this was a very successful meeting.
Details about the 2011 AppInstaller meeting can be read on the FreeDesktop.org Wiki page. There is also a lengthy (53 minute) video that highlights the results of this meeting; the video is embedded below.
Coming out of this meeting is what's being called the AppStream for Linux and it's implementation is already documented on the Wiki. Ubuntu's Software Center will serve as the front-end user-interface after it's been ported to utilize PackageKit. Xapian servers will then power the search and utilize the Open Collaboration Services for some of the social aspects like user ratings and reviews. The AppStream interface will just be a front-end to PackageKit so that each distribution can then continue rely upon their existing package management systems, but the complexities are hidden from the user.