Specifically, when the drop down date picker widget is used to change the date the new value is not propagated to the store until the cell is manually 'blurred'. Which is a problem when the user clicks OK straight away.
There was an issue about DateField not correctly passing its value out when manipulated with the date picker popup, but it has been resolved, see http://www.sencha.com/forum/showthre...-in-3.0.0-RC-2 for more details. As noted there, the fix for this is in SVN, and will be available in the next released.
I've verified this by modifying the example as you described. That said, in the future when filing bugs, simple to run classes implementing EntryPoint are the easiest way to make issues reproducible - setting up a project and working out the IDE and dependencies may result in your issue taking longer to be addressed. We also have a bug template that we ask reported issues to follow, to make reproducing and assigning them a smoother task.