There are several portal-capture programs and applications out on the Internet freely downloadable.
One such application is chillispot, a web portal-capture application that can do some of the things the PublicIP system can do.
Check out
http://www.chillispot.org/ for more information.
Unlike the PublicIP system, which is a LiveCD implementation, you will have to setup a server (or two depending), install the chillispot software, install MySQL for the database server, install Apache-SSL to handle the login securely and install Radius to perform the client authentication and configuration. This is NOT a drop-in solution by any stretch. You will also need to install Perl and PHP so you can handle the administrative side of the equation as well as write the control and reports interface to operation the system.
Just thougtht I would answer the question for you.
Of course, there are other solutions out there - check out M0N0Wall and dd-wrt as they have web portal-capture capabilites built into them and dd-wrt runs on a linksys WRT54 series wireless router (and several other platforms as well).
Hope this info helps...
gm...