The work contributed to your wiki is automatically covered by the author’s own copyright. This means that unless you clearly state differently the individual contributing words and pictures to the wiki and even doing the “layout” owns that content and can decide what to do with it.
In theory this means that they could demand that you remove anything they have added to the wiki.
Because most pages on a wiki are created by groups of people traditional copyright rules have little relevance and no real merit. For this reason most wikis publish under some sort of “copyleft”, Creative Commons or Free Document license.
You will need to choose the right license for your project. The Creative Commons organization has a number of licenses including the Wiki-CC (still in beta) that has been especially adapted for wikis.
This license is “share alike” allowing the ideas and words in your wiki to spread freely (i.e without asking permission or having to pay for it), but requires anyone re-using all or part of the wiki to give an attribution back to the original wiki, rather than to the individual contributors to the wiki.
The on-going discussions about this license can be found here.
When thinking about the appropriate license for your project you should also consider WikiMotivation and WikiOhana.