I was looking at starting up a site that would sell bitcoins through credit card transactions as a US based business. Not wanting to run into immediate regulatory problems I gave FinCEN (Financial Crimes Enforcement Network) a call. It took them a couple of weeks to get back to me but the answer was yes that business would be required to register as a Money Service Business.
Yes, that is expected. BitInstant is a registered MSB, AFAIK. A couple other exchanges based in the US are registered, or in the process of registering.
According to my research (WARNING: not a lawyer, seek your own professional advice) online bitcoin exchanges in the US would need to register in ~47 of 50 states, as well as registering as an MSB with the federal government. (or limit the users accessing the website to a select few states)
There is a multi-state surety bond and registration process, making multi-state compliance a bit easier, but it is definitely a lot of paperwork and background checks required.