I could buy that. If it uses some volatile ram with a batter backup, like the BIOS settings in a typical PC, then:
-When there is power there is no issue, the settings are retained
-When you lose power for some time, the settings are retained by a (relatively) small battery about the diameter of a quarter with part numebr CR2032. That way if you remove your car battery you can still start the car 5 minutes later after replacing it (it remembers your keys id).
-When you lose power for a long time (such as an ECU that was sold off the shelf of a salvage yard and has been unplugged for God knows how long) then the battery is discharged and you lose all your settings.
Now if it uses some kind of NVRAM or SRAM, that doesn't require pulses of current to maintain the memory state then the guy is full of it. I hope for your sake that its something simple like this and it does just in fact need to be reprogrammed again.
I bet there is an in-car secret programming method they're not gonna tell you which associates the key with the ECU... something cryptic like put the ignition in ACC mode and then leave the trunk lid open for 1 minute and turn the left blinker on and then pull the key in an out 100 times or something weird like that. I remember programming my friend's maxima keyless entry was like this. However, in your case it sounds like you have the parts uninstalled from the car so none of that would be possible, so they will clearly hook it up to some computer and reset it.
In any event, good luck.
Fred Allen Burge:Update: The 02 ECU is all wired to the best of my abilities so far. I have an appointment at 1:00 with the local Nissan dealership to have my key reprogrammed to the ECU. Apparently I have a NATS malfunction which basically means the ECU doesn't recognize the key. The leading theory is that the ECU has been out of the donor Maxima for too long and has "forgotten" it's memory so it has to be reprogrammed. I kinda doubt that but we'll see later today.
Hopefully though it will work and I'll have this rig back on the road again for a full review of a stock ECU and functioning VTC's very soon! Then it's time to tidy up the wiring and start tackling the other smaller issues that I've been putting off like finalizing my rad pipes, installing the PS system and getting custom lines made, fixing my bent motor mount, figuring out what I'm going to do about my hood clearance problem, etc.
Fred