We don't have confirmation on this beyond the word of a Czech-based developer who has tested it, but apparently Apple is blocking push notification services in unofficially unlocked iPhones. However, we have tested it and it works.
The developer says the following:
According to technical documentation, every Push application has to request the unique token from the Apple's APNS servers to identify the device it's running on. Thanks to that token, APNS servers always know which device is yours. The token can be understood as an IP address — the server has to know where to send the notification and for which application. APNS can also change your token regularly for higher reliability, so it's critical that the application requests the token again on every start (or when enabling the Push feature) to replace the old one if new token is forced by APNS.
On any unlocked iPhone, the application requesting the token is stuck. APNS does not provide any response at all and the application can either cancel the request completely by automatic timeout or let user wait with the proggress bar forever. Either way, the user will never receive any Push message, because APNS has not provided the token.
However, I have an unlocked iPhone here in Spain and the push notification works perfectly—for example, with AIM. The catch, however, may be that my JesusPhone is using the official Movistar network, not a different one.
Whatever is the case, can Apple legally block these push services to people running iPhones on non-official networks? Would they be interested in doing this on purpose—since unlocked iPhones is such a small percentage of units? Or maybe it's just a technical glitch?
According to the developer, it doesn't matter: They argue that Apple has all the right to block services for these users—since they don't use the official carrier—even while it's hurting its ratings. However, they are asking Apple to provide a way to inform users with unlocked phones about why they don't get push services using third-party applications. [Powerybase via AppAdvice via 9to5]