And does it matter if they're open or closed?
When passports are open, they can be read from a distance of 1 foot, using an RFID reader conforming to regulations in most countries (some nations limit the power output of readers and other devices, in order to avoid having them interfere with each other). You could potentially build a reader exceeding the legal power output, and thus extend read range.
U.S. passports have a lining in their cover that prevents the transponder from being detected by any equipment and at any distance while the passport is closed. However, I am unsure whether every nation that has implemented RFID-enabled passports has put something in the cover to prevent unauthorized detection (I believe most have).
—Mark Roberti, Founder and Editor, RFID Journal