Sooo... given that this is by design. How would one use the WRT1900 with a small office with Windows SBS2010 in place? Normally the clients would use the DHCP of the server, and register with it.
For every other Linksys router we've put in place we always configured the DHCP server on the router to handle it and point the DNS back to the SBS 2010 server, turning off the SBS2010 dhcp (something we can't do here).
In this case...
If I use the router's DHCP server - none of the clients will resolve back to the server (which has a static ip) or any other dynamic resource that is added to the network.
If I disable the routers DHCP and use the server's... I lose the ability to hand out IP's and settings on the wireless side (and the guest network)
For me it appears it is a no win situation and it's time to return this one to get the netgear that does it fine :(