I don't specifically have that model 2Wire model, but I have the 3700HG-V (used in AT&T U-Verse setups). The 2wire Homeportal units seem to detect and know when you're double-NATing (as the Homeportal has a router and Wi-FI AP built-in and your Linksys is one as well) and either warn you about the setup or flat out refuse to work.
I'm not sure if your unit has this, but setup your Linksys as Tyler instructed -- and head to the router's admin page (http://192.168.1.254 for me) and look for an option called DMZplus. This option passes your 2wire's external IP address to a MAC address of your choice (your Linksys router's WAN-side MAC address in this ase) -- so your Linksys router behaves as if it's attached directly without a router in between like your previous cable modem setup. (Technically, it's a layer 3 bridge that passes all incoming traffic to the assigned DMZplus host -- so it's not exactly like a direct modem connection, but pretty close)
This page will also let you control which of the attached devices get which IP (in your case, just the router's WAN-side should be attached to a LAN-side port on the Homeportal).
See if that helps!
EDIT : I missed the part where you said you needed the Homeportal's Wi-Fi capabilities -- in this case you could disable DHCP on your Linksys unit and not attach anything to the WAN port. This way, you can use your Linksys unit as a plain switch to attach multiple computers using the Homeportal's router capability. You can also just use the Homeportal as-is since it features four LAN-facing ports.