Looking at the iPhone SDK beta 4 API change log reveals that the following API calls have been removed from UIApplication.h:

Removed -[UIApplication registerForRemoteNotificationsFromPublisher:]
Removed -[UIApplication registeredRemoteNotificationPublishers]
Removed -[UIApplication remoteNotificationSystemIdentifier]
Removed -[UIApplication remoteNotificationsAreActive]
Removed -[UIApplication unregisterForRemoteNotificationsFromPublisher:]
Removed -[UIApplicationDelegate
application:didReceiveRemoteNotificationFromPublisher:userInfo:]
Removed UIApplication(UIRemoteNotifications)

Also some properties have been removed from CLLocation:

Removed CLLocation.course
Removed CLLocation.speed
Removed CLLocation.speedAvailable
Removed CLLocationDirection
Removed CLLocationSpeed

Removed CLLocationManager.locationServicesApproved

I haven’t installed the 2.1 beta yet but it appears that Apple is removing the ability to receive remote notifications in user applications. This is a feature that many have been eagerly awaiting.

Update

From the beta 4 readme file:

What’s New
Beta release 4 is a bug fix update. This release also removes the client-side APIs for the Apple Push Notification Service for further development.