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.