I didn’t see any mention of this on any recent blogs, so I’m guessing they’re rolling it out slowly? In any case, you can clearly see the “Older Version” link in the screenshot above, and if I click on it, I then have a “Newer Version” option.

So far, I’ve only noticed two differences – the window title is much longer (now “Gmail – Inbox – [EMAIL_ADDRESS]“), and the tooltips for Quick Contacts are a bit fancier. I personally prefer the shorter window title – with the older version, when I’m in another tab, I can tell at a glance if I’ve got new messages simply by whether or not the title for the Gmail tab has a “…” at the end.