2014-03-06 12:26 GMT+01:00 Andreas Schildbach <andreas@schildbach.de>:
 
In current phone implementations, the screen must be on already for NFC
to be active. Also it must be unlocked, although I certainly hope future
OSes will allow payment apps on the lock screen, just like they allow
music players.

Just a small input to this point:
On Android 4.4 the new host card emulation (HCE) feature (aka: the phone emulates a ISO-DEP Smartcard and processes ISO7816-4 APDU commands like a Smartcard would do) only works when the display is on, but even when the screen is locked (can be changed with "android:requireDeviceUnlock" in Manifest). See here for detailled specification: http://developer.android.com/guide/topics/connectivity/nfc/hce.html

Using the HCE API on Android 4.4 also has the beauty that any app that registers itself for HCE and sets its category to CATEGORY_PAYMENT in the Manifest automatically shows up in Adroid's system settings under "Tap & Pay" (where a user would expect payment applications).