We're experiencing the same problems. We have SP2010 with ADFS in place. In an iFrame we're loading an external application which is also configured to use ADFS.
If we place an on the page we can open the external app without logging in so SingleSignOn works.
However if we load the external application in an iframe we experience problems.
If we run Fiddler to see which calls are made we see the "normal" adfs flow. When adfs return with the Redirect to the url of the external app the FedAuth cookies provided are not included in the Post-request to the external app. Since the external app does not recieve the cookies we end up in an infinite loop.
The problem is caused by IE treating the authentication cookie as a third party cookie. It's security settings prevent it from adding the cookie to the request. When we change the IE privacy settings to low (accept all cookies) everything works fine.
Does anyone have any idea how we can fix this?
