The semantic desktop, as implemented by tracker, cannot glean all useful semantic information by indexing alone. This is why there is a separate storage daemon (trackler-store) which apps can use to supply additional semantic info. However even that is not enough to realise its full potential.
So what is needed?
For me, a service orientated architecture (SOA) is needed for objects like contacts, music, images etc. This would span all local objects - eg your locally set up contacts, objects on remote machines (EG contacts on corporate address books like an LDAP server) and of course web service contacts (myspace, facebook etc). An SOA would require a federated database and the obvious candidate for that is tracker
I would also suggest that SOA implemetations be freedesktop based so all desktops can use them and allow KDE/GNOME to replace EDS and Akonadi for a common contact SOA
SOAs would also provide methods for importing/exporting contacts to various services as well as unifying all contacts under one umbrella.
SOAs can go a long way to eliminating duplicated code between desktops and competing applications. In one rosy future, all applications will end up being thin clients with their bulk of their function in SOAa and their data in tracker. Apps could then be created which use any platform (QT/GTK/NBTK/PYGTK/GTK#) with minimal of code. Dont like the UI or the toolkit? No problem, just rewrite it in super quick time
The biggest problem of course is not coding it but getting everyone to tango - it would require a monumental freedesktop effort that would need to overcome all the politics and factions. That sadly means it might not ever happen however...
One of the reasons Im more excited by a desktop variant of Moblin than say Gnome 3 stuff, is that I will be able to implement a tracker based SOA for it as it already comntains SOA stuff like mojito. I do envy the mobile folks as they have the power to forge their own mini desktop environment but who knows. One day soon you may well see a moblin desktop variant where semantic desktops and SOAs will power it.