Assuming you don't have tons of entries per minutes in your wiki, You can also achieve synchronization by using NoSql db CouchDB or Tokyo as a backend. There is also redis. They are replication-friendly.
An experimental solution could be also TiddlyWiki on a Dropbox, really decentralized ! (or maybe too crazy but you can test It :) )
About the Wiki and CMS you can use with those backends Github is the place to search.
really really simple but extendable
simple sinatra couchdb wiki
Instiki (Rails) could be adapted with Ohm
You can also consider sqlite3 asa synchronizable backend since the db It's just a file. Frameworks like Rails and Django support It out of the box.
MORE: After some thinking It comes to my mind that of course you can use git too, since it's a decentralized thing by nature. If you build your wiki on top of git, You can add hooks to every push to fire synchronization with other cloned repositories of your wiki or just luanch a pull every few minutes. And here yuo have an example of a wiki-over-git.
I've found also zim that you could combine, again with git.