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.
Unfortunately the login as other user is unlikely to reappear anytime soon. My recomendation is to use a runas.exe shortcut to launch browser as another user. Less convenient, but more reliable in the long run.
I’ve just posted a blog on the subject. My solution is to create a JavaScript bookmarklet using your URL plus some client object model. A bit more convenient than a runas shortcut but as you say, maybe not as reliable.
http://corypeters.net/2012/10/sharepoint-2013-sign-in-as-another-user/