So I already have a iphone 3G and I could re-up with AT&T for 2 years and get a 3GS discounted but I'm travelling soon and think I'll buy an unlocked 3GS. What are the issues if you want to use an unlocked 3GS on AT&T's network? Is activiation required? Will it be recognised? I want to be able to use the phone both on AT&T and any other GSM providers network here and globally.