Hi, and Welcome to the Forums..
Ok, since you already did a PRAM reset and that didn't resolve the problem.. there is ONE LAST step before we can say it is or could be the logic board.. Do a PMU reset. This is a hard reset where you would have to do the following:
A full (PMU) reset of the Pismo (Apple calls it a 'PowerBook (FireWire)') follows:
If the computer is on, turn it off.
Press and release the reset button located on the rear panel of the computer between the external video and modem ports.
Wait 5 seconds.
Press the Power button to restart the computer.
If and only if you do this and the problem is not resolved, either you will need the Apple Hardware Test which came packaged with the Pismo or you will need MacTest Pro also called: Apple Service Diagnostic to do further diagnostic checking on the Pismo's ethernet port.
Try the PMU reset above and report back.. Try to then plug in an ethernet cable and see if u can get any response from it. If not, I would suggest getting the AHT or MTP from someone.. Actually, if you email me I might be able to further assist as I DO have the tools to make your Pismo work again, hopefullly assuming its not a logic board issue.
Romko