For some reason the docs didn’t tell you how to actually install this plugin, so here are some pointers.
First, if you're reading this more than a month or two after I post it you should go to the svnup home page for up-to-date files.
Grab the following files:
Then copy them to the following places, where $IDEA_HOME is the installation directory of IDEA and $SVN_HOME is the installation directory of subversion:
After a reboot and configuration it seems to work okay, although I've only done some very basic actions so far. It feels a good deal slower than CVS but that could just be IDEA building its caches and such so I'll withhold judgement until I've put a full day in.
Also, just noticed that Metissian has put together disk images for the subversion client and IDEA plugin on OS X -- nice!
UPDATE: I mistyped and had the library 'svnjavahl.jar' going to the '$IDEA_HOME/bin' directory when it should be going to the '$IDEA_HOME/plugins/svn4idea/lib' directory. Fixed.