For some time now over in the US AT&T have held exclusivity over the iPhone; in Canada is was Rogers, while over here in the UK O2 secured iPhone exclusivity, but slowly that is all changing.
AT&T has been lambasted over poor service with the iPhone, but is still the only carrier to have the iPhone, although there is rumour Verizon will eventually gain the handset. Rogers is losing exclusivity as Bell Mobility and Telus both grab a slice of iPhone action making Canadian have a choice of 3 iPhone carriers.
O2 UK has lost exclusivity to Orange and Vodafone which opens up the iPhone market in the UK enabling those who have yet to jump aboard the iPhone train to do so in the not too distant future. The Apple iPhone is spreading now more carriers can offer which is a good for customers and also good for Apple.