It isn't quite a 2.0 release as some blogs claim, as there isn't anything substantial besides speed improvements, a new Contacts form, and a different drop down menu for "More Actions," but Gmail has been updated.
If you don't notice anything new, either you have the new version and didn't realize it, or you'll have to check your top right hand corner and click on "Newer Version." (Some people still don't have it yet, though, as I've confirmed.)
Then you can switch back to the Older Version.
The only noticeable differences for me are the drop-down menu (I'm a heavy Labeler, so I use this often) and the speed -- it looks good. I did get an error yesterday where I was prompted to use Gmail in basic HTML (sadly), but overall, the changes look good.
Now where is Folders support, guys?
Gmail has also written about these code improvements on the Official Gmail Blog.
Forum discussion continues at DigitalPoint Forums.