KB 937333 - Deploying more than 255 Vista systems with WDS (Windows Deployment Services) that automatically join a domain? You'll want this hotfix.

Basically, the Vista WDS client has (had) a hardcoded limit that can be worked around with this patch.

More than 255 Vista systems can be deployed, but only the first 255 will be joined to the domain properly without this patch to the WDS client DLL.

You can read more about it and request the hotfix here.

If you are planning a large rollout, I would request this hotfix beforehand and roll it into your image.