Here's my situation: I have my internet connection provided by a wireless network (we'll call it "A"). I don't have a lot of control over that network, though. I have a wrt610n flashed with dd-wrt, and I'd like to use it as my main router/connection point to that network. I'm fairly sure I need to setup at least one of the antennas as a "client" to network A. I've tried this, it works fine, I get a WAN IP on the router. However, now I'd like to turn around and offer another network (we'll call it "B") for my devices to connect to. And, that means both wired and wireless devices. Can someone help me figure out how to configure it to do this?
You can do this in DD-WRT by setting up a wireless repeater. Essentially you connect your AP as a client to the network (as you've done) and then create a sub-interface on the router which broadcasts a different SSID. That is connected to the internal interfaces of the router enabling you to manage the broadcast network.
The docs on this are pretty comprehensive so there isn't much point in me going through this blow by blow, have a look here: http://www.dd-wrt.com/wiki/index.php/Universal_Wireless_Repeater
Anecdotally I used this to great effect with an 18dB omni directional antenna attached to a WRT54GL while camping in Europe.