From the Google Operating System blog (emphasis mine):
Gmail has a new architecture that improves the performance and the usability. Now you can use the back button in your browser and bookmark URLs from different Gmail views because the URLs change when you go to a different section. The messages are prefetched when Gmail loads so you don’t have to wait too much until a message is displayed.
With all the work I’m doing on Really Simple History, I’m excited to see exactly what kind of back-button support the new Gmail offers. If I use the "newer" and "older" links to navigate through 10 messages, will I be able to use the back button to cycle back through them? Or will only the major views of the page (Inbox, Starred, All Mail, etc.) get stored in the browser’s history? I’m also interested as to why only IE7 and Firefox users will see the new interface. What’s holding up Safari support? (For that matter, what’s been holding up official Opera support all these years?)
If you’re one of the lucky few who’s already gotten to see the upgrade, please tell us all about it in the comments – especially your experience with the back button.
In the meantime, here’s a round-up of posts about the upgrade: