# to install the latest stable version:
brew install scala --with-docs

# to install scala-2.10.0-RC3:
brew install https://raw.github.com/gist/4118557/scala.rb --with-docs

# to switch versions (from https://github.com/mxcl/homebrew/wiki/External-Commands):
brew switch scala 2.9.2
brew switch scala 2.10.0-RC3

# to see which scala you're currently using:
brew which scala