PXE boot requests would need to be directed towards your FOG server. If an imaging task is scheduled, and the client is set to PXE boot, it will then begin the scheduled task. If no task is scheduled, you can have the FOG menu displayed for x seconds before booting into the client OS.
[URL=‘http://www.fogproject.org/wiki/index.php?title=Modifying_existing_DHCP_server_to_work_with_FOG’]Modifying existing DHCP server to work with FOG[/URL]
Definitely check out the FOG wiki when you have the time (bookmark it!). It is a great resource!