Gmail's interface is unique in that the rendering is being done by a Javascript Controller in a hidden iframe. Gmail itself only requests raw data over the wire.
If you just want the look and feel of Gmail, there are plenty of frameworks referenced in the comments (Cappucinno, YUI2, Sencha, etc) that can give you the basic layout. But if you want the mimic the speed/functionality, it'll require getting dirty with Javascript controller design in the browser.
If you just want the look and feel of Gmail, there are plenty of frameworks referenced in the comments (Cappucinno, YUI2, Sencha, etc) that can give you the basic layout. But if you want the mimic the speed/functionality, it'll require getting dirty with Javascript controller design in the browser.