The Chrome OS notification center -- already in the Windows releases of Chromium and Chrome Canary -- is in the process of being ported to the Mac version of Chromium, says
Google's François Beaufort. The Chromium evangelist points to a logged
Chromium issue titled "[Mac][MC] Implement notification updates and relayouts." He comments that the Mac version of the notification center is still "under heavy development," but that it can already be toggled on by running Chrome with an "--enable-rich-notifications" switch.