Originally Posted by NoNameFace
Pretty sure you already know this, so this is more for others (who always use Wifi at home) who may not know; Under Wireless & Networks, go to Wifi Settings and put a tick in the top two boxes for Wifi and Network Notification. If you snag yer phone and head out the door, as soon as you are out of range from your router then 3G will auto kick itself on. If you get close to a public Wifi (Denny's, McDonald's, Starbucks etc.) there will be a new icon on your notification bar to let you know it's there should you want to hook up. When you get back home 3G will turn itself off and your phone will hook back up to your Wifi.
I never have to turn 3G or Wifi on or off. My phone does it for me.
In that situation, I believe your Wifi is still searching for networks which would cause it to drain the battery faster. Your Wifi isn't really off, it just isn't connected to a network.