iPhone gives preference to Wifi wireless connection over GPRS 3G connection when both connections are present, but this is only true when the wireless access point is defined. If the wireless access point is a service that requires you to signon first via a web browser, you can not use the internet without this signon step. You can not choose to connect via GPRS 3G as its all automatic.
Let say you are a regular Starbucks’ Wifi service user. When you walk into Starbucks, iPhone automatically connect you to Starbucks wireless access point. You can not retrieve emails, surf the net or other internet activities without entering your credentials first.
Seamless internet connection experience is possible when you are connected to a ‘friendly’ Wifi access point, such as the one in your home networking setup.
If you are annoyed by the additional signon steps for those public Wifi service, you can remove the particular Wifi access point definition in your iPhone. Once the access point has not been used and defined before, iPhone will select GPRS 3G as the connection path. Please note that iPhone will always prompt you to select the WiFi access point when none of the surrounding Wifi access points are defined. Remember to cancel when prompted if you want the convenience of GPRS 3G connection.
Removing the Wifi access point from the iPhone is easy but not obvious. You can only remove the access point when you are connected to it. Starts the Settings App, select the “Wi-Fi” option. Under “Choose a Network…” section, you should see the Wifi access point you are connected to. Touch the blue color arrow button on the right. This brings you to a screen where a big “Forget this Network” button is on the top. Touch this button to remove the access point from iPhone.