IANAL, but here's a summary of what I've learned watching the GLBSE situation:

Crowdfunding is not legal in the U.S. yet.  The JOBS Act was passed in April of 2012 but the SEC was given 270 days to rule on the actual mechanisms of enforcing the law.  They have dragged their feet.  In August, they came out with a ruling about Title II, but that only affects crowdfunding for accredited investors.  It only lifted the public advertising restrictions to accredited investors.  Title III affects the general public rights.  They have said nothing and probably wont well into next year.

Also, all crowdfunding will go through SEC cleared broker/dealers or designated crowdfunding portals.  You can't just start your own exchange and start crowdfunding anymore than you can run a bank out of your garage.

So current law still applies.  If this forum is considered a public place, and you advertise the sale of shares of a security (which only needs to pass the Howey test to qualify), then you are under the SEC jurisdiction in the US.

If its a private business, then you can raise up to $1 million by selling shares and only need to file a Form D with the SEC.  See a lawyer for all the details.

No matter which path you take, none of these securities can be trade for 2 years or so after they are purchased.  (Someone with more knowledge please correct me).

In the UK, the FSA has the authority over the sale of securities.  In general, if the collecting of funds is a donation, then you are in the clear.  If there is any kind of return of money to the investors, then the FSA has authority.  One unique exception to this rule is if the "securities" are clearly labelled "bets".  I have no idea how that holds up, see a barrister for all the details.

It would seem that "IPO, shares, dividends, etc" is the wrong language all together since it also implies incorporation and other regulations.  These mining contracts are private contracts between individuals.  You are selling computational time to a private citizen.  You probably still want to follow the basics of contract law which lawyers will be more than happy to take your money for.