pfsense would be the quickest, easiest and most secure way to do it. The captive portal feature is very cool. Had a good play with it for a little wifi network in my cul-de-sac. Simple to set up and completely separates your lan from the wlan. Plug a high powered AP in to the pfsense box and it's all done.
Ok, here is what I'm planning to implement. My issue is that I have only one physical Ethernet network so I'll be trying to configure a VPN connection between the Open WiFi Router (dir615) and pfsense server. What do you think?