There might be a way to disable this, but if Wi-Fi is switched on and you find it useful when it finds a network you can actually go on and transfers the internet it can be pretty useful, however if all it finds is some network that doesn't let you on and yet opts out of the 3G signal for that it is rather annoying. Eg I went to a chippy near a library and found I had no internet because the phone had logged on to "Libraries Wi-Fi" which gave no access because you had to register first, couldn't the phone have a way of checking that there is internet access before committing to a Wi-Fi network and turning off internet via 3G?