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/iojs
As 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 --force
If 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.