I've just completed upgrading our Subversion cluster to 1.4.2
I did this because a new customer wanted to mirror their repository to another server, a great idea that we completely support. You can never be too careful with your data!
Subversion 1.4+ includes mirroring support, but both servers must be running the 1.4+, so we happily performed the upgrade.
During the next maintenance window, I'll likely reload the repositories from svnadmin dumps, to update the repositories to the new delta format employed by 1.4+. There's no immediate need to do so as this version is entirely backward compatible with the older format.