I don't own Cosmic Encounter, but I thought the base game was one of the smaller boxes (like the small box AH expansions.) Is that correct? If so I find it odd that an expansion would be a big box.... Just doesn't seem right somehow.
Given that both of the posters who heard this rumour about a new AH expansion are (against all reasonable odds) living in the same area and hearing the rumour from the same store owner, I think it's highly probable that this store owner is simply mistaken. I'm not saying he's deliberately lying, but I think he got his wires crossed somehow. As other have suggested, AH itself is being reprinted, so he probably saw that and mistook it for a new expansion.
As far as the two unknown boxes on the front page, I'm thinking the smaller (non-epic) one is probably Gears of War, which is still slated for Q4 '09 according to the upcoming page, despite apparently still being in the design phase. I don't see GoW being an epic box game, but I suspect it is one of the two unknowns.