IOS 6.1 (out now) fixes the POST caching bug
iOS 6.0 contained a bug that caused Safari to inadvertently cache POST requests. See
http://arstechnica.com/apple/2012/09/developers-claim-safari-in-ios-6-breaks-web-apps-with-aggressive-caching/
This bug has been fixed in iOS 6.1 which was released yesterday. Good news for web applications!