In setting up the Sonic Wall, we wanted Wifi users to be in the same DHCP address range as Ethernet clients. We're using WPA2 and it's nice to have everybody in the lab be on the same subnet.
But try as I might, DHCP wouldn't let me use an overlapping IP addresses / subnet masks, etc. It insisted on two DHCP address ranges. And if I tried to put both X0 and W0 in the same subnet, it complained about that too.
It also was starting to sound like SonicWall expected Wireless users to be on a VPN. Some companies do this - you have a relatively low security or even open Wifi network, but to get to "the good stuff", you have to go onto a VPN. While I understand this rationale, it's overkill for us, and we trust WPA2 (mostly...)
It turns out the answer to both questions is to tell the TZ-201W to bridge the W0 interface to the X0 (LAN) interface. You do this by editing the W0 interface and change it from having a fixed (or dhcp) over to using L2 (layer 2) Bridge, with a target of X0.
This joins the traffic on both inferfaces, and when you look at the dhcp page, you just see one range.
Of course at this point wireless clients have access to your entire internal network, so it's super important to make sure you have proper wireless security set first!
By the way, you do NOT accomplish this by setting the Virtual Access Point Group to Internal AP Group.