I wish I was kidding, but I'm not. Thanks to posts from XDA forum members ald9351 and HCdroid, we now know that the picture AT&T posted with a globe on the back of the device was not just the work of an overzealous graphic artist. Here she is in all her glory misery.
To "boot" (pun intended), Big Blue has put its logo and jingle on the boot screen (displaying for a few seconds before the stock Android boot animation).
AT&T's Nexus 6 Does Indeed Have The Globe On The Back, Plus A Globe And Jingle On The Boot Screen, Because AT&T was written by the awesome team at Android Police.