In the UK, Apple's deal with O2 is coming to an end and both Orange and Vodafone have indicated that they will offer it. T-mobile have said they're sticking with the G2 as their preferred smartphone. Ha, ha, good luck with that[1].

[1] T-mobile are being acquired by Orange so whatever they say about anything is subject to change... Can't imagine their new overlords will be happy about them dragging their heels.

Not always. In Australia, it's available on all the major carriers - Telstra, Optus, Vodafone and 3 plus some virtual carriers who resell service from the majors.

Not sure how common that is outside of the US. In Sweden it's available on all carriers, I think. (It was exclusive to one carrier for the first year or so.)

Here it is offered by most carriers.

Not true, most countries offer both carrier-locked and unsubsidized (unlocked) iPhones.