There's an implementation in the Giki Wiki, as well as a IWCList of IWC-enabled wikis (currently just TheGikiWiki?). See http://giki.sourceforge.net/index.php?title=InterWiki%20Content%20Plugin
The received content would be in rendered HTML (not some wiki syntax), and would simply be placed into the template of your wiki.
To allow retrieval of IWC content, there would be a script called "iwc" in your wiki's directory. Normally this "script" would actually be a directory with index.php or index.py or index.asp (evil) or index.cgi or whatnot in it. This way, the language of the wiki is unimportant.
This script/directory would take HTTP GET arguments:
While this certainly isn't a true inter-wiki standard, it would at least allow for seamless integration of view.
As a last note, there could be multiple ::s. If you ask to get IWCList::TheGikiWiki?::Index, it would ask the IWCList wiki for the title TheGikiWiki?::Index. IWCList would then ask for TheGikiWiki? to give it Index. This way, there could be one or several "meta-wikis" that would serve as lists of other IWC-enabled wikis.
I'm currently writing this as a plugin for GikiWiki? ( http://giki.sourceforge.net/ )