How common are free wifi hotspots?

I’m considering getting an Ipod Touch in part because of the cool wifi features. But I’ve never had a mobile wifi device - how common are public wifi locations, either generally speaking or specifically for Las Vegas?

I get the impression that places like coffee shops, airports, and libraries have them. Are they generally free and open access, or do you need to pay for access? How much does it cost?

Is there a place that has maps of free wifi coverage areas?

Las Vegas-Clark County Library District locations with Wi-Fi