If you're in a country where a dollar is used, and you see $, you assume it's the local dollar. If it's not, they'll use something to denote the currency. If it's the internet, same thing, you see if the company is Canadian, or American, or whathaveyou.
My comment was very specifically showing how using the dollar sign in a country where a local currency which is also a dollar (ie. Canada) but not USD is not confusing, because you have local context. You know you're not talking about the US Dollar, because the local currency takes precedence.
And, as winthrowe replied, what would happen is you add a currency denotation after the value, such as CAD, USD, AUD, etc. It's already done all over the world.