First of all, can you confirm that you are using Build 2266 at version 11.5.1 - if not, use LiveUpdate to get this version as it has full support for Windows 7.
Secondly, can you supply a lot more detail about your Win 7 image and how it was generated? For example, are you running Sysprep before imaging ? How is the machine being joined to your domain?
Once the image is deployed, can you log into it using a local admin account, and if so, does a check in Device Manager show any missing drivers? Clearly if the NIC driver has not loaded for any reason then the machine is not going to join the network. Thus for completeness, can you detail what fault finding steps you have tried so far to avoid us covering old ground.