Here’s an idea that I ran across about a year ago, and I had occassion to suggest it to another friend: if you want to create a wireless network for anybody to use get a cable modem and a wireless access point.
I first saw this at a financial institution that wanted to offer wireless to their customers as they waited in the lobby. However, they didn’t want the wireless access points on their internal networks. So they ordered DSL, bought a Linksys access point, and wrote the WEP key on a white board in the lobby, which they change once a quarter.
They installed the equipment near the lobby in an accessible place, so if it stops working (which sometimes happens) the receptionist can just go power cycle the mess. Aside from being stupidly simple to implement and maintain the system administration staff can now test their corporate VPNs and firewall policies from the comfort of the lobby couches.
I did this for a client. They had two lines, one SDDSL for “corporate” stuff and one ADSL for the guest wireless connection you describe.
The bizarre thing is that pretty much anyone with a laptop VPN-ed into the corporate network from the wifi, because they got better general web-surfing speeds from the ADSL.