Federated (Synced) Subversion servers?

This is more or less the perfect use case for SVK. SVK is a command line front end for subversion that works with an entire local copy of the repository. So your commits, updates, etc. work on the local repository and you can then sync with a master. I would generally recommend SVK over plain subversion anyway as it makes a lot of things nicer. No .svn folders, better branching and merging, better conflict resolution.

Subversion 1.5 introduced write through proxy support for webdav servers over the existing SvnSync support that was added in 1.4. This allows you to have local mirrors for retrieving files and history, but commits are committed directly to the master repository. If setup correctly the local mirrors receive the changes immediately.

See the Svn Book for more details.