I have only seen this when there is MAC address filtration active on the network; if you have not added the MAC addresses to the “approved/allowed” list then they will not be given IP addresses and this is the message that you get.
Are you sure that the machines are in fact communicating with the network (DHCP server) correctly? You could test this by just loading into a vanilla Windows installation from DVD (or Linux Live CD) and see if you have an IP address and can get on the network; if you cannot do this then FOG is not the problem.
I seem to remember imaging E6520’s no problem… but perhaps the network adapter is different.