Well, it has been quite a long time coming but, finally, there is a dictionary plugin for KRunner now. Jason Donenfeld posted in his blog today that he has completed the dictionary plugin for KRunner.
KRunner is one of the reasons (but not the only one) that I love KDE SC. With this new plugin, it just become a lot better. It will not be included in KDE SC 4.5 (since it is supposed to be released tomorrow :p), But, hopefully, it will make it to KDE SC 4.6.
However, for those who cannot wait till KDE SC 4.6, Jason has also posted instructions on how to install it. Here it is:
Open the Konsole and execute the following commands:
(Note: I have tested this in Kubuntu 10.04 running KDE SC 4.5 RC2 and it works without any problem.)
$ svn co svn://anonsvn.kde.org/home/kde/trunk/kdereview/plasma/runners/dictionary dictionary-krunner
$ cd dictionary-krunner
$ cmake . -DCMAKE_INSTALL_PREFIX=/usr
$ sudo make install
$ kquitapp krunner
To start using it, open KRunner with ALT+F2 and type "define" (without the quote, of course) followed by the word you want to know the meaning of.
Update: Jason has added a feature which will allow you to set custom trigger word instead of just "define". Head over to his blog for the details.