If I go to Preferences under Web it shows Wikipedia as the default search engine. However, under Just Type, I have DuckDuckGo selected as the default search engine. If I open the web first (tap on the globe) and then type in my search, it does go to Wikipedia. However, if I go directly to Just Type (enter my search in Just Type on the home screen), it goes to DuckDuckGo.
I have no idea why the web Preferences differ from the Just Type selection. Just repeat to yourself the following: It's not a bug, it's a feature. If you say it enough, you'll start to believe it.
In the meantime, select Bing as the default under Just Type and start your search from Just Type rather than opening the web first. You can also change out the web selection on the home page (I have DuckDuckGo rather than the default globe web app on home page) so you start off at Bing if you do want to open the web first.
I don't know if this was a change that happened with 3.0.5--I updated last night so I can't check at this point.
If you want to use the Just Type function once you already have the web open, I'm not sure how you can get Bing on there. If you open up Just Type again, and look under the Search Using section, uncheck Wikipedia (and Twitter) and leave Google checked. Your first Just Type from the home page will be in Bing. Once the web is open, anything after that will be in Google...but at least you won't get Wikipedia...