http://www.fincen.gov/financial_institutions/msb/As far the US is concerned, these guys make the rules, which are quite simple: If you, the currency issuer, are offering to sell that currency for government sanctioned currency (legal tender from any UN aknowledge nation as far as I can tell), you need to be a MSB. People trading it for legal tender in your back is none of your concern as long as you make it clear in your rules and the EULA you have your users approve that you forbid such practices.
Corollary to this rule, if you make yourself into a Market Maker, i.e. offering a platform that allows for any currency to be traded for legal tender class currencies, you need to be a MSB, whether you're issuing any of this currency or not.
Don't quote me on this but I think these rules don't apply to commodities, i.e. gold, silver and the like. Those depend on other regulatory bodies entirely.
I have no idea about regulations outside of the US. In a general fashion, no one but the US gives a shit about virtual currencies. Most of these, Bitcoin included, aren't even legally acknowledged as currency outside of the US.
The FinCen 2013 move on virtual currency doesn't count as regulation, only as guidance, however I'd think twice about opening a business that falls under their criteria and skip the MSB licensing. Big gov and all, you know. Keep in mind that getting a MSB is a Herculean task.
This post was edited by flyinggoat on Sep 24 2013 10:26pm