ので、ソッコー入れた。
$ brew update
remote: Counting objects: 1886, done.
remote: Compressing objects: 100% (790/790), done.
remote: Total 1645 (delta 1216), reused 1223 (delta 844)
Receiving objects: 100% (1645/1645), 279.88 KiB | 171 KiB/s, done.
Resolving deltas: 100% (1216/1216), completed with 204 local objects.
From https://github.com/mxcl/homebrew
   84a2e44..1c1f1ad  master     -> origin/master
Automatic merge went well; stopped before committing as requested
Already up-to-date.

$ brew info svn
subversion 1.7.2
http://subversion.apache.org/
Depends on: pkg-config, neon, sqlite
/usr/local/Cellar/subversion/1.6.17 (90 files, 6.0M)
http://github.com/mxcl/homebrew/commits/master/Library/Formula/subversion.rb

$ brew install svn
Error: subversion already installed
Try: brew upgrade subversion

$ brew  upgrade subversion