Add the following DHCP Scope Options:
066 IP Address of WDS Server
These are required steps if your DHCP Server is on a different subnet than your SCCM/WDS/PXE Server.
Here is are some good KB Articles on this subject – http://support.microsoft.com/default.aspx/kb/926172 and http://technet.microsoft.com/en-us/library/cc732351.aspx
Stolen from Paul Jones Notes from the Field.