I found a tool that downloads Wikipedia database dumps and converts Wikipedia into a Dict file: It's called Wik2dict and I got the feeling that it will prove useful when stress-testing the planned Dict-file plugin for DictionaryReader ;-)

Update 2013: I've been using Aard Dictionary for offline Wikipedia access now; it works quite nicely, and if your phone has an SD card slot, it's also feasible to carry around. :)