If you read the list, it is clear that no one is disputing the facts of what happened; no one is making unfounded assumptions. Rising Tide Systems has a SCSI module that they have written entirely from scratch. In the process of writing this they pushed much of it into the kernel, so much in fact that one of their employees became the Linux SCSI targets maintainer. They have kept some of it back and are shipping a modified kernel containing their code to customers without providing the source code.
They believe that this is allowed, and that their code is not a derivative work of the kernel. Basically RTS is saying: If NVIDIA can have proprietary drivers, why can't we have proprietary kernel subsystems? The other side believes that what NVIDIA is doing likely is a GPL violation, and furthermore some of the technicalities that NVIDIA claims make it legal don't apply to what RTS is doing.
This issue has been ticking time bomb ready to go off. It is entirely possible that some, all, or none of the proprietary drivers written are a violation of the GPL. It all depends on the courts interpretation of derivative work, and no one knows for certain (although some arguments have stronger precedent than others). Furthermore, it is too late to add GPL linking exception to Linux's license to clarify this (one way or the other), because there are too contributors at this point to come to an agreement. So it will remain murky as mud until someone finally sues someone over the issue. Sounds like this may actually be happening. But even then, the ruling may end up depending on the nature of the proprietary extension, and thus remain fuzzy.