MacPorts told me that there had been a subversion update (1.7.1), which I went ahead and installed. Woo! Huge speed improvements for everything I tried with the CLI client, great stuff.
A short time later my IDE (PHPStorm) fell over screaming. It doesn't like 1.7 yet, and it's a bit stuck until SVNKit supports it.
I should have checked really. So how to downgrade? Fortunately this post makes it very easy.