Interstitial screen/marketing layer on Airport guest wifi
What you're looking for is a Captive Portal.

You can't create captive portals solely with an AirPort Extreme—you need at least a computer behind the network set up as DCHP/DNS/etc. You're going to need to configure a Mac to provide DNS to redirect the first visit from a device to a hosted site.

For more information about creating a captive portal yourself, see:


http://brewstersoft.com/blog/setting-up-a-captive-portal.html


Commercial software is available which can set this up for you:


http://patronsoft.com/firstspot/