We use Google Apps for domains, including Gmail. Some of our users have gotten the new look forced upon them already, but others still have a choice. I have one person who was given the choice, tried the new look, and then used the Revert Temporarily link to go back to the old interface. He now wants to use the new look, but he is not getting the link to switch. I'm assuming, since the URL doesn't change when you change from old to new, that there's a cookie stored somewhere that lets Gmail know which interface to display. What is that cookie? 
How does Gmail store the new look flag?
gmail
New look switch is stored on Google's servers independently of the browser.As far as I understood from Google's emails to the domains I manage, until the domain rolls out new look to everyone (through a check-box in Email settings in the management console) users should be able to switch between the two. In any case, the re-design will be mandatory within the next 6 weeks (Google recommends voluntary transition by the end of December).