The project's goal is to collect a list of iPhone (and Android) compatible pay-as-you-go (PAYG) SIMs from around the world. Since the card is pre-paid, they're useful for someone who will be traveling for a brief time. To make the list, a card must be capable of handling data as well as voice.
For example, in France you can get an Orange card called "Mobicarte," which provides 30 days of unlimited data for 12 euros, or a subscription from LFR called "La carte" which provides unlimited data for 9.90 euros. Voice and text are charge separately on a graduated scale.
Of course, the iPhone 4 has a micro-SIM, and they're more difficult to find on the PYAG shelves. Fortunately, there are ways to make a full-sized SIM fit. We posted about one method not too long ago. Basically, you can buy a kit to help you physically cut a standard SIM down to size. There's also a hardware cutter that resembles a stapler, which all but guarantees a proper cut every time. Plus, it comes with an adapter that will let you re-use the diced card with a full-sized device.
Some SIMs are locked to a given provider. The wiki provides information on how to deal with that, too. The whole thing seems well-curated and timely. Add it to your travel plans and avoid coming home to an iPhone bill equal to one month's salary.
[Via Nik F.]