I've only ever heard the rumor referring to the Carquest area - that's been brought up for a couple of years now - I personally can't see it - the topography is all wrong for something of that size. I've never heard reference to it going near Costco.
I know conceptual drawings often change drastically but East Point's site plan shows a number of smaller lots going in there - although, I'm sure they would change that in a heart beat for a tenant like Walmart.
I still think the large empty area near the corner of Rothesay Ave and McAllister Dr (near CarQuest's old location) would be a great spot for Walmart - lots of space. Let's start a rumor about that location.