I just started encountering this problem too! It seems a part of Apple's site might be temporarily down, as it wasn't happening earlier. Of course, now I'm wondering what is it about connecting to WiFi that Apple is transmitting to itself.
Same here. It keeps losing the wifi signal and switching to 3G/4G and then a weird Apple.com page comes up with "Log In" in the title bar but no log in form. The only way to stop it is to turn of Cellular Data.
Not even disabling 3G fixes it, for me it comes up with http://www.apple.com in the header, with a page not found error. Tried 2 different wifi networks, connects for a second and then drops it. Also. iTunes App Store seems to be down.