iPhone 5 on Verizon is not locked and you can do this if you desire. I'm not sure about other phones.
That said, the slot is unlocked on the verizon model, so you can pop another micro SIM in there for travel use.
http://www.zdnet.com/nano-sim-redux-using-the-iphone-5-in-th...
when the smaller iPhone sims came out a couple years back there was a backlog on pay and go for a few months as they were reserved for their iPhone contract customers, same will happen with these tiny sims.
(I know that says iPad, so I'm extrapolating)
edit: Yes they have Pay and Go nano for iPhone 5 as well: http://store.three.co.uk/view/searchSimOnly?tariff=112