Something I never knew until just now... http://osxdaily.com/2012/05/24/use-airplane-mode-with-wi-fi-bluetooth-enabled-on-an-iphone-or-ipad/ This can be handy if, for example, you want calls to go straight to voicemail without ringing so that people think "your phone is off," but you still want to surf the 'net or play Words. Or perhaps you need to maximize your battery life for surfing the 'net, and you simply don't need to have your cellular radio on.
Yep, it is pretty handy. I first learned about it a few months after I got my 3G in 2008. I use it a lot. It's nice when you're in an area with WiFi and a poor cellular signal since the battery will drain as it's searching for a signal. It's also nice when using Skype since Skype automatically ends your call if a cellular call comes in.
Got some incredible usage times by using this trick! This includes a very small amount of time on regular (all radios on) mode.
This is true, if you really want to save some battery life while using wifi, it's better to go on Airplane mode or Do not Disturb. I do that sometimes
"Do Not Disturb" by itself helps a negligible amount when it comes to saving battery (e.g., instead of producing a sound, your phone remains silent, therefore saving a couple billionths of a mAh). You will want to engage Airplane Mode to turn off your cellular radio and the phone's signal-seeking behavior.
