Mac OS X Lion Safari Doesn't Display Tab Bar in Full Screen Mode
If you work with keybord mainly try Cmd + Shift + Arrow Keys or [ and ]. You should be able to cycle your tabs like that. I dont have the Developer Preview, so I dont know if there is any quick fix, and I dont think Apple will change it in its final version. After all this is how all major browsers work in fullscreen mode: they all hide the tab bar so you focus on the content... :/ 

Anyway, Hope it helps!
Kevin