The KDE project has started some collaboration with Wikimedia to provide KDE applications with a remote API for accessing Wikimedia databases. Nice ideas have been proposed here.
Since the API seems to going to be desktop-neutral, I guess we need to pay attention to start using it in GNOME as soon as it is available. gnome-dictionary could use Wkimedia sites as its sources, Beagle could search in Wikimedia articles, Totem/Rhythmbox could display information on tracks/artists/albums as they play music/video, text editors could allow importing of those articles, etc, etc.
In case you were not already aware of it Abiword arleady includes a basic Wikipedia lookup tool which is kinda neet.
http://www.abisource.com/twiki/bin/view/Abiword/PluginMatrix