ZDNET's David Meyer reports that you can modify an existing SIM (or micro-SIM) to nano-SIM proportions with a pair of household scissors, a little patience and a steady hand. Several websites have noted clipped SIMs (and micro-SIMs) work just fine in the iPhone 5, although it's an oft-debated question.
Probably the easiest way to do the deed, is to follow this instructional PDF from iPhone APN Changer. There is one important wrinkle to consider before busting out your trusty Fiskars:
Measuring 12.3 x 8.8 mm, the Nano-SIM is about 30 percent smaller than the Micro-SIM. Even the thickness (0.7 mm) of the card has been reduced by about 15 percent – a tremendous technical challenge.
As noted by Giesecke & Devrient (PDF) the potential issue for DIY-ers is that proper nano-SIMs are 12-15 percent thinner than their predecessors -- which is definitely not as simple as just trimming around the edges. The good news is that the iPhone 5 appears to accept the thicker, clipped nano-SIM "without any sanding."
If you're worried about backwards compatibility with your new, svelte nano-SIM in older devices NANOSIM SHOP sells a number of "German made" nano-to-micro and nano-to-SIM adapter sleds starting at €3.99 plus €2.99 shipping.