You don't need a separate currency since you can just create a company that will sell gold for bitcoins.
Someone selling gold for bitcoins at a set price is not a promise but only a temporary promise to exchange gold for bitcoins at that price and that temporary promise can change at any time. If bitcoins were backed by gold there would be a permanent promise to always sell gold for bitcoins at a set price. A temporary promise that can change at any time is very different from a permanent promise. Someone selling bitcoins for gold is very different than someone backing bitcoins with gold.
What incentive would whoever is backing the currency have to back the currency? Would they just do it out of the goodness of their heart? If you want to start backing bitcoins with gold, nobody is stopping you. But I can't think of any reason anyone would do that.
I agree there would need to be incentive for someone to assume the responsibility of backing goldcoins with gold. The 0.2% per year storage fee and the 3% above spot auction fee discussed in my original post are areas that have potential for profit as it is pretty easy to store gold for less than 0.2% per year and also pretty easy to acquire gold for less than 3% above spot (i.e. the maximum over spot goldmoney charges is 2.49% and the maximum yearly storage fee is 0.18%). However, the largest potential source of profit is the likelihood that many auctions will be for more what would be required to back the goldcoins. For example, if an auction of 1000 goldcoins brought in $2000 and it only cost $1500 to purchase the necessary ounce of gold to back the goldcoins then that would be a $500 profit. There is plenty of incentive for someone to create and back something like Goldcoin.
It is just another pegging scheme.
It is not a pegging scheme because although you would have a guarantee that 1000 goldcoins gets you an ounce of gold, an ounce of gold is not guaranteed to get you 1000 goldcoins.
... if miners are spending their money on gold instead of electricity and hashinh-hardware the blockchain might be quite vulnerable to attack by anyone who chooses to invest in computing gear instead of gold.
The Goldcoin system proposed would be just as vulnerable to attack as Bitcoin will be after all the bitcoins have been issued because Goldcoin would use the same transaction fee system as Bitcoin. I think the Bitcoin transaction fee system is a good model for Goldcoin to follow.