This issue has been fixed for the next release!
The loading mask no longer automatically appears when store.load() is called.
The loading mask spinner with loading mask text used to work in PR3 and no longer works in PR4. You can reproduce this problem using the twitter and kiva examples that ship with PR4. You will see the background mask appear and then disappear after the records are loaded, but no spinner will be displayed.
This was purposely disable in PR4 due to an issue with centering the spinner on scrollable containers. This has been resolved and will be in the next release.