Are you seeing this even after the Mac has been booted up and running for at least a minute or two? I ask because unfortunately there are cases where the network interface (built in or external dongle) doesn't get initialized until a few moments after the Mac is up. its generally more prevalent of an issue with external dongles, but the problem can happen even on built in Ethernet like on iMacs. What happens if you unplug the network cable and plug it back in?
Also, what is your imaging process like? Assuming you're imaging these Macs, its possible some network related settings from the Mac you created your base image from was left in the image, which can cause problems when applied to new systems.
