I was told by an Apple employee yesterday in the Lakeside store that I could pop in on launch day and they would give me a nano-sim for my Orange contract if they have some spare.
Orange UK also told me on their webchat that they will have some in stock for existing customers by launch day and that I needed to phone them up on the 21st to order one.
Simon
Various Orange UK CS persons have said various stories in various places!
I would go with the Apple Store, but "if they have some spare" sounds ominous! Sound like you need to join the Q around Monday night!