Update (Jan 2019): io.js has already been merged into Node.js since v4, and is now considered deprecated. Consider using nodejs instead.
An updated homebrew-iojs formula is maintained by @aredridel and is available with just one command.
brew install aredridel/iojs/iojsAs of iojs 2.0.2, Homebrew ships with its own formula for iojs. This isn't preferrable, in my opinion, since global npm packages aren't being ran properly.
brew install iojs
brew unlink node
brew link iojs --forceIf you'd like to use this anyway, simply doing brew install iojs is not enough. To get iojs working on Homebrew You will need to unlink node first then use link --force to install iojs.