Java 7 JRE Not Detected on Clean Install of Mountain Lion
In Oracle's JRE 7 for Mac installation instructions, it states:


  Installing a JRE from Oracle will not:
  
  
  Update java -version symlinks or add java to your path. 
  Show the installed JRE in the Java Preferences.app 
  
  
  To be able to do the above, 
   you need to install the JDK.


After installing the JDK, go to Applications > Utilities > Java Preferences.app and drag Java SE 7 to the top to indicate that it is preferred.  You may also switch off Java SE 6 entirely without uninstalling it.