Exciting days with the release of SharePoint 2013 beta. First thing i noticed on a setup on one of my servers, is that you do no longer see the option to sign in as a different user. Apparently the link is either missing or has willingly been removed from the UI. A little comparison with a SP2010 environment shows that the actual link for signing in as a different user is /_layouts/closeConnection.aspx?loginasanotheruser=true.
Fortunately the link still works in SP2013, which might make me believe that the link has just been forgotten adn will probably be back in the RTM release. If not, I guess the very first customization a lot of customers will ask for is to have the button back in the User menu.