Referring specifically to the discrete graphics cards market, the researcher said AMD commanded 51% of shipments during the quarter while Nvidia grabbed a 49% share. Again, just last year they were at 41% and 59% respectively. There are a number of reasons for this role-reversal, starting with the fact that AMD released its first DirectX 11 cards several months before Nvidia. Also, it probably didn't help that the GeForce GTX 480 and 470 cards were heavily criticized for their noise and heat output, while their performance wasn't a great deal better than AMD's offerings.
It should be interesting to see if the trend maintains next quarter with both manufacturers expected to have a fair range of DirectX 11 cards on the market. The GeForce GTX 460 was certainly a step in the right direction for Nvidia.