This will be done soon (we'll go with Attribution-ShareAlike probably). Thanks for pointing that out.
As being an active member or bot Wikipedia and OpenStreetMap, possibly the two largest open content projects in the world it is obvious that I come up with the content licensing terms of OpiWiki.
Right now - as far as I (don't) see it - licensing is undefined which - by law - results defaulting to full copyright which is extremely unfortunate.
As a license I would prefer an established open content license (my advice is CreativeCommons Attribution or Attribution-ShareAlike), apart from the website icons and graphical elements.
To help this there should be an easy and obvious way to link to a given content or graph which would satisfy attribution requirements (as far as I see that's already done, just mentioning it).