On my Android 4.0.4 (ICS) Xperia ray, they are stored in `/mnt/asec/XXX/pkg.apk`. 
`XXX` is the [system name](http://android.stackexchange.com/a/28155/18442 "I don't know the exact 'android' term for this folder") of the application.  
  For example, Firefox is found at `/mnt/asec/org.mozilla.firefox-1/pkg.apk` and Skype is found at `/mnt/asec/com.skype.raider-1/pkg.apk`.