According to a comment reply on the Disney Parks Blog, the hitchhiking ghost mug IS expected to be sold in limited quantities at WDW. But, nobody knows when. I would guess closer to Halloween. From photos taken at yesterday's first Trader Sam's luncheon for the Food and Wine Festival, they do actually have at least some of the mugs, because one of the included drinks was served in them. Guests were not allowed to keep them or purchase them, though.
Note that in
Disneyland they had lines of several hours to purchase these mugs, and they sold out the first day, so anybody who wants one will likely have to arrange to spend a chunk of time over there.
I was actually at the Grog Grotto on October 30 last year, and they did not have any sort of Halloween merchandise, nor did they have a special Halloween event like Disneyland has.