I'm concerned that if we wait until the wiki contains many substantial articles and then try to impose a copyright license it may cause friction with existing contributors if they disagree with the specific license chosen. OTOH I don't think we should rush into choosing a specific license, so may I suggest that we (==Grunff as sysadmin :-)) configure our mediawiki to put in a placeholder copyright notice, under the edit box on edit pages.
Maybe we could say something like "Contents of this Wiki will be subject to a license yet to be determined but possibly like GFDL or Creative Commons. If you are unhappy with the license eventually chosen you will be able to edit your content out of the current article but it may not be practicable to remove it from the system (i.e. it may still be accessible under the page 'history'). Please consider this when deciding whether to contribute content."
[Sorry, this is a drive-by posting: I'm going to be away from the Internet (gasp!) for a few days.]