Forms Authentication in MVC 3 fails in Safari / production

I'm using Forms Authentication in MVC 3 and all works great on my development wo开发者_JAVA百科rkstation, but when I post to the production environment authentication fails... but ONLY for Safari. It works great in IE, Firefox and Chrome, but not Safari.

Any pointer would be most appreciated!

EDIT:

It seems that setting a non-persistent cookie works:

FormsAuthentication.SetAuthCookie(user, false);

So this would appear to be something to do with the expiry date?

Anything particular about expiry date and Safari?

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ASP.NET Forms Authentication with Windows Safari