The day following the unplanned free-to-play weekend brought on by server troubles, which took down the purchasing process, saw sales of 25,936 copies and 48,800 registrations. In the last week, the game has sold over 80,000 copies. At around $13 a copy, this amounts to around $1.1 million in the last seven days.
It doesn't hurt that Penny Arcade mentioned and created two comics about the game, as well. Congratulations on the success, Persson. Good luck with the company. I'm pretty sure Valve will buy it at some point.