As of Android Studio 0.8.4 .gitignore file is generated automatically when starting new project. By default it contains:
.gradle
/local.properties
/.idea/workspace.xml
/.idea/libraries
.DS_Store
build/
/captures
I agree with this statement, however I modify this file to change /build to build/ (This will include /build and /app/build) So I don't end up with all the files in app/build in my repository.
Note also that if you import a project from Eclipse, the .gitignore won't be copied, or "automagically" created for you.