Can you force an iPhone to use wifi instead of 3G?
Question: Can you configure an iphone 4 so that it always uses a wireless internet connection for web access when one is available, as opposed to using the iphone's built-in 3g internet access (and being charged for it on your mobile plan)?
Answer: iPhones are already set up to automatically connect to "known" wireless networks when one is available. This means that once you've connected to (joined) a wifi network then your iPhone will remember that network in the future, and will automatically use the wifi network instead of 3g.
You can also set your iphone to ask you to join any networks it sees, giving you the choice to join or ignore those networks. This can be a little annoying if you're roaming around in an area where there are a lot of wifi access points, so I usually turn that off.
To configure your wifi settings, tap "Settings" and then "Wi-Fi". You'll see an option to "Ask to Join Networks" at the bottom. Turn this setting on and you'll have full control - you'll get notified when there's a wireless network available, and you'll get to choose whether you want to join it or not.