There are only 21 million bitcoins out there, so ANYONE could back bitcoins with Gold. If you have 21000oz, you can give out an oz for every 1000 bitcoins.
Uh... I could today. Maybe this week. But I won't make an indefinite guarantee. And neither will anyone else.
I don't think the value of a dollar is arbitrary. It certainly has a floor because 300 million people in the US find it directly usable, unlike Bitcoin. Until you can pay taxes in Bitcoin, it will be completely speculative.
Everything is speculative. The history of fiat currency since long before the Chinese invented paper is an inauspicious tale. Dollars will decompose, it's just a matter of when.