Basically money transmitters are regulated as MSBs. MSBs require registration by FinCEN and they are considering people who mine Bitcoins for US dollars as such.
"A person that creates units of this convertible virtual currency and uses it to purchase real or virtual goods and services is a user of the convertible virtual currency and not subject to regulation as a money transmitter.
By contrast, a person that creates units of convertible virtual currency and sells those units to another person for real currency or its equivalent is engaged in transmission to another location and is a money transmitter. In addition, a person is an exchanger and a money transmitter if the person accepts such de-centralized convertible virtual currency from one person and transmits it to another person as part of the acceptance and transfer of currency, funds, or other value that substitutes for currency."
A person is defined as this, even if they are an MSB:
"FinCEN’s regulations define “person” as “an individual, a corporation, a partnership, a trust or estate, a joint stock company, an association, a syndicate, joint venture, or other unincorporated organization or group, an Indian Tribe (as that term is defined in the Indian Gaming Regulatory Act), and all entities cognizable as legal personalities.”
http://www.fincen.gov/statutes_regs/guidance/html/FIN-2013-G001.html