I've never been inside a "Pismo", but on the tray-loading iMacs which use a similar sort of CPU card the main system controller (MPC106) is indeed located on the processor card. (In essence making entire motherboard little more than a collection of PCI peripherals.) Given how the Pismo CPU card likewise has the RAM slots on it it's a fair bet it also has the UNI-N system controller on it. And if that's the case then a bad connection could certainly cause all sorts of havoc with anything sitting on the PCI bus, and that would include the Cardbus bridge IC.
(On the flip side, however, I would think that a problem like that would probably pretty much brick the whole system at once but I suppose it's *possible* that the Cardbus controller is more demanding of the bus than the USB controller.)
You do know, of course, that all the "CPU upgrade cards" they sell for the Pismo are just Apple cards with the original CPU melted off and a new one soldered to the same board, right? That's why the upgrades don't go any faster than a 500Mhz G4 or the 900Mhz G3s; those are the fastest CPUs which are pin-compatible with the stock G3. In principle a manufacturer could make faster cards by melting all the support chips off the stock boards and transferring them to a new-build card but given the difficulty in reusing surface-mount components I suspect such a board would cost a mint. (The other possibility would be to insert some sort of adapter/carrier between the BGA pads on the stock card and the new CPU, but that's also "really hard".) Parts like the UNI-N aren't available on the open market, so a full new-build card is basically an impossibility. (If they were available then systems like those ridonkulous "PowerPC Amiga" machines that people keep trying to build wouldn't be so eternally stuck as vaporware. The biggest hurdle the makers of those things have to face is that nobody but Apple made a good desktop-oriented PPC Northbridge. Motorola/Freescale didn't bother because their primary non-Apple customers were embedded system manufacturers. Network routers don't need AGP.)
(On the flip side, however, I would think that a problem like that would probably pretty much brick the whole system at once but I suppose it's *possible* that the Cardbus controller is more demanding of the bus than the USB controller.)
You do know, of course, that all the "CPU upgrade cards" they sell for the Pismo are just Apple cards with the original CPU melted off and a new one soldered to the same board, right? That's why the upgrades don't go any faster than a 500Mhz G4 or the 900Mhz G3s; those are the fastest CPUs which are pin-compatible with the stock G3. In principle a manufacturer could make faster cards by melting all the support chips off the stock boards and transferring them to a new-build card but given the difficulty in reusing surface-mount components I suspect such a board would cost a mint. (The other possibility would be to insert some sort of adapter/carrier between the BGA pads on the stock card and the new CPU, but that's also "really hard".) Parts like the UNI-N aren't available on the open market, so a full new-build card is basically an impossibility. (If they were available then systems like those ridonkulous "PowerPC Amiga" machines that people keep trying to build wouldn't be so eternally stuck as vaporware. The biggest hurdle the makers of those things have to face is that nobody but Apple made a good desktop-oriented PPC Northbridge. Motorola/Freescale didn't bother because their primary non-Apple customers were embedded system manufacturers. Network routers don't need AGP.)