The person I spoke with had this to say:
There are no limits which exclude you from registering when dealing with virtual currency. However:
If you are buying/selling from a site such as mtgox.com, who (currently or soon?) will have coinlab which is registered with FinCen, IF you are limited to buying/Selling for speculative purposes (buying, and selling later for gain), AND as a user (trading for goods):
Then you would have the position of a speculator and would not need to register with FinCEN. MtGox/coinlab would have the responsibility in this case to report transactions to FinCEN, since they are registered.
It is only if you decide to branch out and buy/sell directly from other people that you would need to register, as well as be in compliance with their regulations.
Just a summary of what I was told by FinCEN, I can't be responsible for you following this info but seems to be accurate based on what I was told.
------------------Update 4/1/2013--------------------
I talked to Anthony from FinCEN again for further clarifications on our discussion.
It's not so much whether the other party is registered by FinCEN but it's what you are doing with the coins that matters. Basically, if you're only buying/selling coins for speculative, investment purposes then you are a user and don't need to register. That could include buying/selling from another person directly, or buying/selling physical casascius or virtual coins on ebay. If you do the ability to show that it is only for investment purposes, and use as a "user" would be on you if there were any questions.
If you're doing this as part of a business (which includes mining coins probably, since mining leads to profits of income), then you would need to register. Some points about registering, though:
-It's free to register with FinCEN
-You CAN, in fact, be your own compliance officer. He said that many business owners do just that.
-Was pointed to the following page:
http://www.ecfr.gov/cgi-bin/retrieveECFR?gp=&SID=8540ec934b849dc3f899a9454b6f32d4&n=31y3.1.6.1.6&r=PART&ty=HTMLHe explained that for the most part, the only things that need to be reported are suspicious transactions, and you can search the web for examples of sample anti-money laundering programs. I asked what forms of identification we needed to collect from those we did business with, he replied that I should read the regulations but in general, MSB don't have the same guidelines, as say, banks do. My interpretation of the guidelines found at
http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&SID=b99714739ce703375951af7f060e38ef&rgn=div8&view=text&node=31:3.1.6.1.2.3.3.7&idno=31 is that you only need to verify the identity of the person when the amounts exceed certain thresholds.
I technically am registered as a sole proprietor, so I might or might not register. It honestly doesn't seem to be a big deal after actually talking through it and reading the stuff. I do not know, however, how the state level will impact it as of now.
------------------Update 4/3/2013--------------------
Anthony had this to clarify after checking with his colleagues:
An exchanger is a person engaged as a business in the exchange of virtual currency for real currency, funds, or other virtual currency. An administrator is a person engaged as a business in issuing (putting into circulation) a virtual currency , and who has the authority to redeem (to withdraw from circulation) such virtual currency.
SO
Even if you obtain the bitcoins as part of a business, if you are not in the "business in the exchange of virtual currency for real currency, funds, or other virtual currency" and are just selling coins obtained from a user standpoint, including from a business such as sale of items, etc, then that does not require you to register just because you exchange for USD later.
I think this latest clarification will make a great number of users of bitcoin not needing to register. However, if miners are still required to register, I still don't know how transacting with sites such as mtgox will work.