The short story is that all the G84 and G86 parts are bad. Period. No exceptions. All of them, mobile and desktop, use the exact same ASIC, so expect them to go south in inordinate numbers as well. There are caveats however, and we will detail those in a bit.
Laptops get turned on and off many times in a day, and due to the power management, throttle down much more than desktops. This has them going through the heat cycle multiple times in a day, whereas desktops typically get turned on and off once a day, sometimes left on for weeks at a time. Failures like this are typically on a bell curve, so they start out slow, build up, then tail off.
Since laptops and desktops have a different "customer use patterns", they are at different points on the bell curve. Laptops have got to the, "we can't bury this anymore" point, desktops haven't, but they will - guaranteed. The biggest question is whether or not they will be under warranty at that point, not whether or not they are defective. They are.
Now you can take the Inquirer with a large pinch of salt if you wish, but last week when news broke of this issue I followed the trail of destruction that that led me to conclude that the GeForce 8400M was a likely suspect. More digging, along with news of an update from HP made me add the GeForce 8600M to the list of suspects. If some are affected, it's not a big leap to assume that all (or a large proportion) are affected, making this a very big problem indeed for NVIDIA. Want to dig the hole even deeper? Well, consider that these GPUs have also been used in countless notebooks, including Apple's MacBook and MacBook Pro lines. If this really turns out to be this big, then $200 million isn't going to go very far.
The way I see it this kind of problem was almost inevitable. Both NVIDIA and AMD/ATI are responsible for pushing higher and higher voltages through the silicon and relying increasingly on the coolers to move more and more heat away from the GPU. Any weakness in the thermal system (joints, thermal grease, epoxy, fan performance ...) is going to lead to problems, and even a small design flaw is capable of becoming a huge problem down the line.
Until NVIDIA come clean on this issue (and according to one expert I've spoken to, it could take months to fully get to the bottom of this problem) uncertainty will cloud the company.