You will have to write your own service to notify of updates. You can use a listener to push the update to the phone where it can be automatically downloaded by the listener, but I believe you will need to have the user manually hit the "install" button to update the app since it's not coming from the marketplace. Also, depending on the app configuration you may need to uninstall and reinstall the application on update, but unfortunately I don't have details or instances where that would have to be the solution.
Also, keep in mind that AT&T phones don't (out of the box) allow for installing apps not from the marketplace. They are said to be working on a solution for this, but it might only be for the Amazon App Store.
Edit: You will also need to make sure the phones the app is installed on has "Unknown Sources" checked in Settings>Applications
