The more we play the more gold we pull out of those dungeons. Spare gold will likely find its way to the AH after a while unless there is a fair amount of upkeep causing us to have to use it. Eventually the supply will be pretty huge. On top of that, what happens to all of the gold that Blizz scraps off the gold AH? Will blizzard sell it? Will it sell theirs before yours? I haven't seen anything stating Blizzard's plan for their gold or how they plan on managing the gold economy.
Seems to me that a huge never ending supply of gold will eventually overtake demand and the price of 100,000,000,000 gold(or what ever the max is) will drop below $250. When this happens items prices will balance in both AHs and the advantage of the gold AH disappears. Then your awesomesupercool_ginormouscost item can reap you your $250. Or... maybe you look for another market place.... just sayin.