Joel has a lot of what look like good recommendations. However, I can provide a much-abridged list.
Some years, WiFi access at PyCon US has been provided by the venue, or by a contractor whose name I mercifully do not know. Those years, it has not worked. Some years, it has been provided, or at least managed, by tummy.com. Those years, it has worked. They are probably much more critical of their own efforts than I am, as you can see in this thorough write-up that they did of PyCon's 2008 WiFi situation.
My two-step plan for you if you want your conference to have working WiFi access at your conference is:
- e-mail somebody at tummy.com, telling them that you want a working wireless network, and
- give them whatever they ask for.
If you do these things, then when people open their laptops at your conference, their networks will work.