It's not speculation which the liberty reserve case clearly shows. If you don't verify customers using "imaginary internet money" you are a target of the US. The reason is that users could be from the US using a VPN etc to access your service, if you don't verify customers you have no way of ensuring that. So you MUST, with no exception, verify all customers. All business must do this, and those who don't will risk ending up like liberty reserve.
edit: and if you don't deny all US customers you will also have to become licensed in the US. If you are fully licensed in the US (is mtgox?) you may be able to skip the legitimation requirement in a few cases, but I'm not sure about that.