Gold is always useless in these types of games because even if gold only dropped 1% of the time, you'll be killing hundreds of thousands of monsters over your Diablo "career". Item selling will probably be a problem, though, especially early on where people haven't had much chance to find the great items.
Bashiok said that gold will have a value, like in WoW, therefore I'm sure there will be sweatshops mining gold to sell for $5 a thousand or whatever just like on there.