It makes perfect sense since the Nandroid backup is an image of your Droid at the time of the backup... some system checks that triggers that OTA has been imaged back with the nandroid.OTA update notification is back again. Again, this is after a nandroid backup. Coincidence? I think not!
Anyone know why a nandroid backup would cause the OTA update notification to return?
If you want it to stop, then make a new backup, right after restoring that nandroid backup and allowing that OTA to fail... then you'll have a nandroid that's exactly the same 'except' the OTA is no longer going to find those system checks that'll trigger that OTA again...
To be clear, I have allowed it to fail twice before, but performing a nandroid backup (not a restore) brings the notification back.