I won't claim to be the expert, but I'm pretty sure that Guest login is a different "security" from your WLAN security. Whatever settings you use on your WLAN (WPA/WEP/etc.) must be configured in advance on the client in order for the user to connect to your wireless network and get an IP address. Once connected, the user will open up a browser (which will get redirected) to the guest logon page. At this point, the client can only resolve DNS and get to your guest logon web page. Once logged on, the user will be able to actually get somewhere on your wireless network, but the WLAN settings remain the same before, during, and after guest logon.
Also - if you can implement WCS, WCS will allow you to print or email the credentials.