The .app package that all Mac users install from the minecraft.net website will always look for Java 6 and nothing else.
-If Java 6 exists, it will run both the launcher and the client using Java 6, regardless of whether Java 7 is installed or not.
-If Java 6 does not exist, it will either fail to open in OS X 10.8, OR it will prompt to install Java 6 from Apple in OS X 10.9. I don't know what happens in anything below 10.8.
However, Minecraft (launcher + client) runs perfectly fine with Java 7. In order to force the use of Java 7, open the Bootstrap.jar file located in Minecraft.app/Contents/Resources/Java/Bootstrap.jar
This actually looks for the latest Jar Launcher.
If you go to this website http://java.com/en/download/index.jsp to download Java 7, you only get the browser plugin for applets, so that is not sufficient to run Minecraft in Java 7. You must actually install the JDK from here http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html in order to get Minecraft to run in Java 7.
I don't know the Mac sales numbers, but this issue is forcing potentially millions of Mac users to use outdated versions of Java for literally no reason.