/usr/local on a normal, stock OS X install is completely unused. Homebrew's installer commandeers it by `chown`ing it to the current user and makes it group writable, as to cut down on the sudo noise. I think issues like this, and the fact that nearly all package managers, including aptitude, yum and ports require sudo should be catalysts for requiring sudo for updating and installation of packages. It's that whole security vs. convenience tradeoff again.

I've actually run into a similar issue with another installer via a Rakefile that lacked uninstalling capabilities and a confusing method of determining the $PREFIX which resulted in me shredding my /usr/local/bin directory for a couple seconds before my ^C spamming stopped the process.