WiFi gateway and Captive portal

Hello,
I'm looking to set up a WiFi gateway with two nics, and iptables. The
catch is i have to use a captive portal as well to ensure all traffic goes
to a single destination. I've read about several packages, but haven't seen
any docs that say how to integrate everything. If anyone has this setup i'd
appreciate knowing what software your using, and experiences with it,
positive or negative.
Thanks.
Dave.