As you may have heard, there will be minor differences between the Australian and US/EU versions of Grand Theft Auto IV in order to comply with the AU rating system. What you probably didn't know is that this may have caused GTAIV to be region locked.

According to Kotaku AU, many people have emailed Rockstar asking about the lock, and several have gotten the following response :

..All copies of Grand Theft Auto IV are region locked for their specific territories.

Also, while there are some minor differences between the Australian and US/EU versions, they are not significant in the scope and detail that make GTA IV such an incredible experience...

Region locked for their specific territories? Not good for people who may have imported, and not good for anyone if this happens to effect multiplayer. While this isn't confirmed by the "press release"  standards of being official, it does seem to be true. Although by the looks of the email, it seems (and we hope), this will only effect playing the actual disc, and not restrict any online multiplayer.

Source : Kotaku Australia

Also, you may have noticed I'm a new name around here. Well I've been friends with Jared for quite some time now, and also run the fansite GTAUnderground.com. Anyway, greetings. I hope to enhance the site anyway I can.

Update: It has been brought to our attention that the PlayStation 3 does not have a region locking function, therefore the above statement only affects the Xbox 360 version of the game.