I am not an expert on security issues, but I'll lend my opinion to you.

First of all, if you want to offer WiFi in your cafe, consider outsourcing the provision of the service to a 3rd party company such as http://www.whotspot.com, which specializes in this. If you want to offer WiFi access yourself and it is going to be for free, simply segment your network with a router and firewall. On one network you can use it for your own Internet access and on the other network open it to the public and provide visitors with your security key to access the WiFi network. I hope this helps.

Ramon Ray
Editor & Technology Evangelist
Smallbiztechnology.com