There used to be a check box that would allow me to mark questions as "community wiki", below the editing area. I can no longer find that checkbox. This is consistent across all sites that I have tried (StackOverflow, SuperUser, meta.so, bicycles.se). It is consistent across several browsers: Google Chrome 6, Safari 5, Firefox 3.6 on Mac OS X 10.5, and Google Chrome 8 (dev channel) on Mac OS X 10.6.
The "community wiki" checkbox still shows up on answers, just not on questions, either for initially asking the question or for editing it. I have plenty of rep on all sites involved.