Looking a little deeper, ATI has twice NVIDIA's market share in the "performance" graphics market, though NVIDIA tripled its sales in that segment for the third quarter. ATI is way out ahead in the standalone mobile graphics market, too. With a 71% share of mobile GPUs, the Mobility Radeons are stomping NVIDIA's GeForce Go products. The silver lining for NVIDIA is the DirectX 9 market, where GeForce FX cards have a 66% overall share and 72% of the value DX9 segment. However, ATI's new Radeon 9600 SE could challenge NVIDIA's budget DX9 dominance in upcoming quarters, especially since the SE bundles a voucher for Half-Life 2.
Of course, market share numbers don't necessarily reflect the quality of each company's graphics offerings; anyone that's tried to play games with Intel's Extreme Graphics can tell you that. However, market share numbers do influence game developers looking to make their games playable for the widest possible audience.