Logout and flashdata

[eluser]WanWizard[/eluser]
My point exactly.

Destroying a session is (almost) never a good idea. Use a properly defined structure, and delete only that.

For ExiteCMS, the session contains a variable called 'auth', which is an array with all kind of authentication related information. If this variable exists when the user requests a page, the user session is restored. If not, a guest session is started. When a user logs out, I simply delete the 'auth' variable from the session.
All other session variables are untouched, and survive login/logout actions (for example user preferences, selection or filter criteria, etc).