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
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