Does the U.S. Securities and Exchange Commission govern Bitcoin? Do SEC regulations apply to:


Exchange of Bitcoin for US dollars
Investing in companies with Bitcoin
An exchange that sells shares of company stock for Bitcoin
Bitcoin futures contracts
Is a Bitcoin a security?   The SEC hasn't said that it believes it is and not communicated with or taken action, formal or informal, asserting that a miner is issuing securities or that exchanges are engaging as brokers of a security, which is a regulated activity.

Is a Bitcoin an asset or commodity?   Does your grocery store need SEC approval to sell bananas?   Or, more similarly, does a coin dealer need SEC approval to sell gold and silver bullion?

Now a broker where gold bullion is speculated on is regulated in the U.S., just not by the SEC.  The Commodity Futures Trading Commission (CFTC) is the regulatory body that addresses that.  To-date, there are no bitcoin-related services offering futures contracts, options, or anything similar which operate from the U.S. 

Now issuing shares in a company is exactly the activity the SEC addresses.  There are no bitcoin-related services issuing company securities or operating a secondary market for shares that operates from within the U.S.  The regulators may still assert jurisdiction though if securities are offered to investors from the U.S.

Now there may be registration requirements for an organization that holds deposits or where an exchange interacts with the banking system they may need to register as a money transmitter, but that is not something under the SEC's purview.