It used to be possible for client Macs to 'find' the Open Directory server purely via DHCP. With this configuration a client Mac would boot, do a DHCP request to get an IP address, and then would ask the DHCP server for DHCP option code 95 and then use that information to be able to login to network accounts. If you are still trying to use this approach it now works apprently by booting, then using Bonjour to find the Open Directory server, and then adding it to the list. This will take longer.
Even back in the day this was the 'standard' configuration, a client Mac would often not immediately be able to connect to Open Directory and network accounts. These days it is far more common for a Mac to be initially setup with an entry for the Open Directory server which no longer requires using DHCP option code 95. This is done either in System Preferences - Users & Groups, or Directory Utility, or via the command line using dsconfigldap, or as part of a Mac image deployment system like DeployStudio.
With pre-defined entries for the Open Directory server, the client Mac boots, again asks via DHCP for an IP address but is then able to continue its startup and use the previously entered Open Directory server details making the process in theory quicker.
My own experience is that an initial boot of a Mac after its previous DHCP lease has expired will result in the Mac booting, getting an IP address but perhaps for a few seconds not recognising the availability of network accounts. If the only local account is a single admin account then it used to be that it would then go straight to asking for the password for that local account, now in Lion/Mountain Lion it merely highlights that account and when the network accounts are detected you see an arrow pointing left. If there is more than one account available you will initially see a list of local (including mobile) accounts, and after a short delay the 'other' account for logging in as a network user.
Doing a reboot, it usually shows the network accounts straight away as the previous DHCP lease is still valid.
Note: Yes, if your managed network switch has SPF turned on this will typically cause a delay in clients getting DHCP assigned IP addresses, in the case of NetGear up to 30 seconds. As long as your network is properly setup to avoid loops then it should be safe to turn SPF off.