I found this software that you install probably on a server that has access to the WAP that will show a splash website to any users that connect to the WAP. It's called TollBooth and here is what they say on their website:
TollBooth is a service available to our customers supporting wireless HotSpot locations around the world. This service does not require any special software or hardware at the location, instead, operates using standard “off-the-shelf” Access Control devices to control access to the public Internet at each location. With TollBooth, our customers can quickly create 100% custom branded web pages that are displayed when users attempt to access the public internet at a HotSpot location, retail plans which are presented to the users who can sign-up for service using the most popular plans (Monthly Subscription; PayGo; and Pre-Paid Card), process Credit Card payments, and provides a “Customer Self-care Portal” where users can go to self-administer their user account. Administrators of the TollBooth system can control the system conveniently from a web browser anywhere in the world.
skotman:
I feel like it has something to do with proxy redirecting traffic also, or maybe packet inspection?