When Can This Happen?
Companies that plan to asteroid mine are planning to do it in mid or late 2013 and early 2014. Deep Space Industries and Planetary Resources announced their ideas in mid 2012.
Citations
http://kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf
http://www.bbc.co.uk/news/science-environment-17827347
http://lightyears.blogs.cnn.com/2012/04/24/companies-plan-to-mine-precious-metals-in-space/?hpt=hp_t3
http://en.wikipedia.org/wiki/Asteroid_mining
http://news.discovery.com/space/asteroids-meteors-meteorites/could-asteroid-mining-drive-21st-century-space-industry-130204.htm
http://www.bbc.co.uk/news/science-environment-17827347
http://lightyears.blogs.cnn.com/2012/04/24/companies-plan-to-mine-precious-metals-in-space/?hpt=hp_t3
http://en.wikipedia.org/wiki/Asteroid_mining
http://news.discovery.com/space/asteroids-meteors-meteorites/could-asteroid-mining-drive-21st-century-space-industry-130204.htm