Yes I have checked the IsAuthenticated Property and it says that it is actually authenticated.
The way I sign a user out is by redirected them to the home page of the site. On page_load it checks for a query string called signoff, if available it logs the user off. Then the following code is ran:
formsauthentication.signout()
response.write(user.identity.isauthenticated)
So first the user is logged off, then it is printed on the screen wether or not the user is still logged in. The print value is "True." But after reloading the page or visiting another page, the value goes back to "False." Very strange... I do not want to say that it is a bug yet...