brew install android-sdk
brew install scala sbt #scala only
curl https://raw.github.com/n8han/conscript/master/setup.sh | sh #scala only
~/bin/cs n8han/giter8 #scala only
export ANDROID_HOME=/usr/local/Cellar/android-sdk/r16 #ideally add this to your .zshrc or .bash_profile
next run 'android' from a terminal. The Android SDK installer should launch and start to download the list of SDKs it can install. You'll want to install Android SDK tools, Android SDK Platform-tools, Android 2.3.3 SDK Platform and Google APIs underneath Android 2.3.3. E.g. http://imgur.com/s9Y8m (they say Installed here because I've already installed them). Click 'Install Packages' after checking their boxes.
If you want to use an IDE then I'd suggest IntelliJ 11 Community Edition http://www.jetbrains.com/idea/download/
If you're going to use Scala then you should also install the Scala and SBT plugins. (IntelliJ IDEA -> Preferences -> Plugins -> Browse Repositories).
An IDE isn't necessary for this, especially if you're going to use Scala but I'll be using IntelliJ for the initial walkthrough.
If you plan on using IntelliJ then also install go here and do what it says (this will let you generate an IntelliJ project file from SBT) https://github.com/mpeltonen/sbt-idea
If you want to just start where the walkthrough will stop then clone this: https://github.com/jbrechtel/android-hack-night
http://www.nevercertain.com/2012/01/02/android-scala-installer.html