You know the problem, you go to an event expecting wifi, but there is none. But there is a simple solution that both provides a public good and is great guerrilla marketing.
I always travel with an Airport Express configured as a open access hotspot named Socialtext. For a couple of events without wifi, including the one I am sitting at now, I find a rogue port and offer free wifi. People seem to love this.
One time the event organizer kept announcing on the stage that wifi was sponsored by us. Not bad for $100 and couple of ounces to lug around.
Normally I wouldn't blog such a great idea for competitors to copy, but I hope they and others do.