There is a gray area here.
If you are wholly in
BTCitcoin, you can probably sell shares in your virtual (not real) company, as long as it's not really a company (especially in the USA). The gray area comes in if the SEC takes note of you. If you don't have your ducks in a row when they do, it could be very costly. Getting your ducks in a row could be very costly also.
It is probably legal to offer shares personally (without a company behind it). It seems to be legal to offer shares privately (not openly listed on a market) if your company is incorporated in a state, but closely held and not registered with the SEC. Some states have very strict laws (in an attempt to protect citizens / investors) others have few laws (with the idea of allowing the market to be less constrained - like Wyoming.)
If you'll notice, ASICMiner (as a great instance) is a "
BTCitcoin publicly owned company", but it is based in China (I believe). So the SEC has no jurisdiction and if bad things happen to your investment in it, you are not "protected."
There's a fairly lengthy discussion about US securities laws here:
https://bitcointalk.org/index.php?topic=217709.0Recommended reading.
Remember, IANAL, however I've created and run a few companies in my life. Never a
BTCitcoin funded company, but I'm looking at it and interested.