Hi, is your system language set to english? And what is the location set in system preferences? I've discovered that if I set the system language to english and the location to US, wiki results will start working (I'm italian and live in Germany). My theory is that wiki results would work if system language is set to english and the country set in system preferences is english speaking (would make sense, at least for wikipedia results).
Also, did you notice if the SpotlightNetHelper process is or has been not responding?