Since the iPhone 3GS Jailbreak has become available we have heard people are having problems with Push Notification.
Either the Push Notification is not working, or not working properly. The Dev Team have noted they still have some remaining 3.0 jailbreak issues to investigate including push notification, especially as Purplera 1n and redsn0w are now coming on line with iPhone 3GS Jailbreak solutions.
If you already have the new 3GS phone, done a full USB dump, captured your img3’s, signed with your ECID then you are fine, you should always be able to jailbreak. If you are looking to buy the 3GS iPhone you will not already have an ECID (unique chip ID).The jailbreak requires at least one signed iBoot-family img3 for your device and needs to have an exploitable bug, you either have your signed exploitable iBoot ready to use or you have nothing. Look here for more information.
Personally,I think that iPhone manufacturers should add some detailed instructions about Push Notification service.So iphone users will be familar with it and use it more easily.