In my original proposal I suggested that images on the commons would have to be referenced as
[[Image:co:Airplane.jpg]]
I realize now that this is not necessary. We can instead just use
[[Image:Airplane.jpg]]
If you would add this instruction on the English Wikipedia, and Airplane.jpg exists locally, it would use that one. If it doesn't, but it exists on the Commons server, it would use the remote version instead.
This makes the use of the Commons transparent. There should be links on the image (really file) pages to quickly move files to and fro.
We can even provide this functionality for other MediaWiki sites, or wikis which implement the necessary interface. Say you would type
[[Image:Paris-Metro.jpg]]
on Wikitravel; again, if the image exists locally, the local version is displayed, if it does not, the remote version is downloaded and cached for future use (compare timestamps when using cached commons images).
This would also make it relatively easy for a mirror to use our images without downloading our entire collection of them. They would just have to enable that option, and images would be transparently fetched (and updated!) when needed.
As an additional advantage, we no longer need to worry about the fair use issue. Sites which want to allow fair use would download a tarball of these images generated from the files on the *local* wiki (e.g. English), the others would only accept images that are stored on the commons server, guaranteed to be under a free license.
The same principle could even be applied to articles. If you link to [[Declaration of Independence]] from a Commons-connected wiki, and that page does not exist, the wiki would fetch it from the Commons and enter it into the edit box that is normally blanked (like Wikinfo's Wikipedia import interface). You can then copy and paste content from there as wikisource [*], or you can save it locally to get a copy of that page.
What would be really neat, of course, would be transparent transclusion of articles from the Commons:
{{commons:Declaration of Indepdence>p2s1}}
becomes:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness.
p2s1=paragraph 2, sentence 1 - this only works for pages which rarely change, of course.
Look Alice, we're in Xanadu! Again, these segments could be stored locally and compared with timestamps from the Commons on demand.
This is advanced stuff, but could certainly be entered into our roadmap. Any wiki which would implement our interfaces could transclude material when needed.
Regards,
Erik
[*] I actually typed "wikisource" without thinking, referring to wiki source code, not to the Wikisource project. I think that illustrates my point about the name being ambiguous nicely.