One potential explanation for a lack of significant market share growth could be the timing of ATI's R3x0-derived products. ATI's line of R3x0-based graphics cards have delivered incredible performance in their respective price ranges, but with few of today's most recent games taking advantage of the available horsepower, few may be compelled to upgrade. Virtually no applications take advantage of the R3x0's advanced DirectX 9 features, which probably isn't helping matters, either.
Just because ATI hasn't experienced huge market share growth of late doesn't mean that the R3x0 has gone underappreciated. ATI's recent success may not have bought significant market share gains, but it's undoubtedly won a lot of mind share. As gamers look to upgrade their systems to handle the new Doom, mind share could become quite an important factor. ATI has executed well with its latest Radeons, which bodes well for more appearances in future systems from big OEMs like Dell, who currently offer the Radeon 9800 Pro.