One of Curiosity's predecessors, Mer-A, "Spirit," got stuck in a dune in 2009, and died there a few months later.
The dune Curiosity faced lay across a narrow valley named "Dingo Gap." Here are views from orbit and from the ground:
Compounding the problem, Curiosity's wheels have already taken a beating. They've developed cracks and fissures, which reduce the wheels' flotation effects, and also are worrisome for the rover's long-term mobility.The wheels are sustaining damage far faster than anticipated.
So, Curiosity paused for a while at the foot of the dunes while its operators weighed the risks and rewards; and tested the edges of the dunes to gauge the firmness.
But all went well, and the rover trundled across the dunes, as this GIF shows (rearward-facing cameras):
Here's the view from the far side of Dingo Gap, looking back at the dune:
Lots more: http://www.planetary.org/blogs/emily-lakdawalla/2014/curiosity-update-sols.html