some history for clarification:
the iPhone 2G unlock is different from the iPhone 3G and 3Gs unlock, in that, it exploits a hardware vulnerability to unlock the phone to any network, on 3G and 3Gs, the unlock happens in the software of the phone baseband (or modem for practical purposes), this however, can be prevented with further iPhone firmware updates, therefore, the unlock on 3G and 3Gs CANNOT be guaranteed, as apple has the ability to undo and prevent any developed unlock for the phone
jailbreaking again, is a different story, dev team has confirmed that, as long as you have the files required for the jailbreak (which you HAVE to get by restoring the 3Gs to 3.0 firmware before doing any kind of update on the phone), then the 3Gs will always be jailbreakable, however, once the 3Gs releases with 3.1 already installed, you're stuffed there, dev team might be able to develop a jailbreak, but again, not guaranteed
as for push notifications
apple owns the only sever cluster that enable push notifications, for the phone to access this server cluster, there are certain conditions, the most troublesome of which is, only when iTunes activates the phone (which can only be done with a valid sim if the phone is network locked), will the push server cluster allow access to that phone (it uses the phone's unique UDID, not the sim imei or phone number or any ohter ID), meaning, if iTunes does not activate the phone, no push for your app store apps
so again
3Gs bought in USA or any other network locked country:
can't use push notifications
can be completely disabled by any future firmware upgrade, turning it into a very expensive ipod touch
really guys, stay away from iPhones not bought here, one thing to be said about our government, they try to look after the consumers, no network operator in SA is allowed to sell a network locked phone, ALL phones have to be usable on any cellular network, meaning that, you will even be able to activate your iphone with an mtn sim (I activated an iPhone 3G with an mtn sim using iTunes when 3.0 was released, so I can confirm this)