The Mars Science Laboratory (MSL for short) was supposed to launch in the fall of 2009, but has been delayed until the next time Mars is in the best position for launching - 2011. Basically, the rover isn't quite ready yet, and it's expensive. Better to have more time to get it ready, then have a failure on Mars or even worse - a new crater with the Cooper Mini sized rover in a heap at the bottom. Kind of a bummer, but I guess it's better this way. Maybe Spirit and Opportunity will still be wandering around until the next launch?