Google has been rolling out a Gmail 2.0 update for the past few days. I just got the update a few minutes ago, and the first thing I noticed was that every view now has a permalink! The address bar updates as you navigate the application. Each e-mail thread has its own URL, and each label and folder has its own URL. Even searches have unique URLs.
Google is accomplishing this by using anchor permalinks that are able to be dynamically updated without a page refresh.
Individual message: http://mail.google.com/mail/#inbox/1ef02abc56fe3127
The search one will be especially useful, as browser search boxes and tools such as Quicksilver will now be able to point to Gmail search results.
Unfortunately the new version of Gmail seems to have serious issues with Safari 3.0 in OS X Leopard — so there are clearly some bugs to be worked out yet.