Just had the same problem you had. When connectiong to my usual wifi my iPhone / iPad brings me to Apple.com page for a login. If I cancel this, wifi switch off.
Luckly, I found a workaround. Very easy!
When you’re prompted to Apple.com just locate the iPhone section: One link under refers to apps in Apple store. Just go to that link and than chose whatever app. Once you’re in an app page, press the link to buy it in the App Store.
It’s done! You’re shifted to you App Store app, out of settings with wifi enabled.
Now, waiting for Apple fix, you can still enjoy you’re wifi connection!
Hope this can be’ helpful, for every question you can tweet me at @fedino82
Enjoy iOS6! 😉