To make up for the lack of multi tasking in the iPhone, Apple launched “push notifications” as the only other option. Your iPhone receives a message from the server that a new event has occured in any given program and you will be notified accordingly, so it’s a bit like background processed – but then again, not really.
But if you have a jailkbroken/hacked (unlocked) iPhone, you need to be careful with those push notifications. You may end up receiving push notifications sent to other users – and other users might receive push notifications sent to you.
In order for those push notifications to be delivered correctly, each iPhone gets a unique number, kinda like a phone number. This ensures that push notifications are sent to the correct phone, like chat messages. It now seems that this unique number system breaks if you hack your iPhone by unlocking it. It’s quite simple actually, your iPhone is said to have the same “unique” number as some other unlocked iPhones, so push notifications that was intended to you, will go out to others as well.
The developers behind most of the unlocking methods for the iPhone, “iPhone Dev Team”, is said to be working on a solution already.
This is another reason why I wanted a factory unlocked iPhone 3GS…