Possibly related EU/UK legislation, which might affect an exchange in the EU when issuing USD coupons similar to MtGox.

"electronic money issuers will not be allowed to grant interest or other benefits related to the length of time e-money is held"
http://www.fsa.gov.uk/smallfirms/resources/one_minute_guides/systems_controls/e_money.shtml

You also have to be authorised if you take deposits, in the UK & the US. http://en.wikipedia.org/wiki/Non-banking_financial_company No idea about Japan / Singapore.