They outline a very convincing argument based on the rumored history of the R400 project. For those who haven't heard, the rumor is that ATI started work on a next-gen chip, then canceled it and decided to do an upgrade to the R360 (found in the Radeon 9800XT) instead. Hence, R420 will be an updated R360 with a die shrink and more transistors. Those extra transistors are likely to be dedicated to additional vertex shader units on the chip, improved pixel shaders, and err.. other stuff. 3DCenter thinks maybe additional texture units per pixel pipe could be in the works.
I'm persuaded by this 3DCenter's argument because it jibes with ATI's basic approach to graphics since the R300. I explored the basics of that approach in this article some time back. Pixel Shader 3.0 adds things like improved flow control, longer shader program instruction limits, and more arbitrary instruction mixes. PS 3.0 also moves toward a unified pixel/vertex shader model without taking the final steps. So PS 3.0 is elegant, but not necessary. PS 3.0 doesn't add precision or new visual output capabilities. ATI might do just as well rendering more complex shader programslikely compiled from High-Level Shader Language programsinto multi-pass operations on R3x0 and R420, using an F-buffer to store results between passes. Since the first DX9 chips were in the works, NVIDIA has been a proponent of more complex shaders and more CPU-like instruction pipelines, while ATI got a faster chip to market sooner by keeping things simple.
I think this rumor also might explain Microsoft's apparent reluctance to name the next DX9 release DirectX 9.1. ATI may be pushing them to call it 9.0c in order to preserve "DirectX 9 support" as the primary checkbox feature for GPUs.
Intriguing stuff, and the timber for many a future flame war, if these rumors prove true.