My company allows flight mode during the whole flight for any kind of smartphone, pda or similar device, which covers the iPhone or even the aircraft mobile phone. Therefore it is fine to use just that. As FE Hoppy said it needs active user action to power on WLAN, it is off by default when switching to flight mode.
I always put my kindle just to the normal sleep mode, not completely off, no interference actually if you switched off the network services (3g only in my case) since the device is basicly doing nothing.