Google didn't just unveil a new inbox for Gmail; it rolled out an even bigger change. There's a new favicon and while, quite obviously, it's not a huge departure from the old one, it's a worthwhile update.The new favicon is cleaner and better resembles the icons used by the Gmail apps, like the Chrome app for example.
Strangely though, the new favicon is a bit less visible, though the “M” on the envelope stands out better. The new favicon seems to be rolling out to all users, so you should be getting it soon if you don't have it already.
Of course, most people would be hard pressed to spot the difference and would probably not even notice when it changes, especially a minor change like this. Then again, Google can't really go wild with one of the most iconic logos it owns.