I am buying an iPhone 5 from Apple. If it's unlocked will it always be?

yes phones which are unlocked by offcial ways will stay unlocked forever

I guess it would depend upon some factors. What country are you in and are you buying your iPhone in that country? From what I understand, if you buy an iPhone directly from an Apple store in Canada, the iphone is unlocked, but it will automatically lock itself to the carrier which the first SIM it is activated with. I'm not sure about other countries...

Not entirely correct. If you buy a phone from Apple in Canada and specifically ask for an unlocked iPhone, it will always be unlocked. Some stores such as CarPhoneWarehouse in the UK sell "SIM free" iPhones, which lock to the carrier whose SIM is used to first activate the Phone. Only APple sells unlocked phones in most couNtries.

Assuming you are talking UK then yes Apple sell unlocked iPhones

in Store(Apple that is) or on Apple online

You will pay full price for the iPhone and it will work with any UK carrier's nano sim

Generally and certainly in UK iPhones are only locked if buying from a carrier where

they have subsidised the cost of the unit which includes the Carphone Warehouse and Phones for U

to name two

an sim unlocked phone is able to work with every network which use supported frequencies and have namo sims

and will continue to do so until the device breaks

An SIM-free unlocked iPhone bought directly from Apple will remain unlocked forever. However, a SIM-free iPhone bought from Carphone Warehouse here in the UK, will be locked to the first network for which a SIM is inserted.

Carphone Warehouse sells iPhone's on all the available networks. So, instead of stocking multiple network models of the same phone for all the different UK networks (5+) they just stock ones that can be sold for any network but that will lock to the SIM supplied with the phone.

This seems to be a unique setup with Carphone Warehouse (and perhaps Phones 4U) in the UK only.