In V6 and if using Forms Based Authentication you will have to Authenticate a 2nd time under the following scenario's:
1. Using Search or Archive Explorer
2. Retrieving content from a Public folder archived item.
3. If the View Mode on the Policy has been set to use Enterprise Vault and not OWA.
4. If there is a problem restoring the item into the mailbox and it falls back to using the Enterprise Vault V5 type view mode.
Normal viewing of an archived item will not require a re-authentication.
Cheers