Space Law: Is Asteroid Mining Legal?

Can a private company claim ownership of an asteroid based on sending a probe out to it? Can it at least get exclusive mining rights? Would it own the gold, platinum or other materials mined from the asteroid?
Last week, a new private company, Planetary Resources announced an ambitious plan to prospect for and eventually mine near-Earth asteroids. Backed in part by Google execs Larry Page and Eric Schmidt, this venture has stirred the pot once again on the question of outer space property rights.
Understanding the legality of asteroid mining starts with the 1967 Outer Space Treaty. Some might argue the treaty bans all space property rights, citing Article II:
Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.
Others have argued that because Article II only applies to nations, individuals are free to claim chunks of the solar system. But as we’ve noted before, the treaty also requires nations to ensure their citizens comply with the other provisions of the Outer Space Treaty — including a prohibition against sovereign claims of property rights. So neither nations nor individuals can appropriate territory in space. But what about asteroid mining?