Google started rolling out a nifty update to Gmail with the Card UI earlier today, but since the update is staged, it may take a while to get to a device near you. Not to worry - we've gotten a hold of the APK (thanks, Kevin!) and verified it's indeed legitimate. You can find it below on several mirrors. As always, just download and install - simple as that.
Gmail 4.6 also changes the multi-select UI to use checkmarks and tells you if sync is off.
Update: Some users are reporting bootloops after enabling auto-sync with the new update. You may want to hold off installing it for now until Google figures out what's wrong.
Download (Android 4.0+):