Just today, some people started seeing the new version of Gmail -- the improvements (like being much faster) are mostly under the hood, but there are a few noticeable changes.
The most obvious is the improved contact manager. I haven't had a chance to actually play with the new interface yet, but it seems intuitive just by looking at the screenshots.
The Google Talk contact popup has also gone under the knife and looks a lot cleaner.
Overall, the change isn't major from a users perspective, but it is still welcome. The Gmail interface has remained largely untouched since it was launched -- and maybe that's a good thing?