Availability is still the biggest issue. Nextel runs a WIFI network and has nodes in every Starbucks and one of the major bookstore chains (can't remember which one) but you have to be in proximity to one of them to access the network. Most of the other major telcos have setups to so you really have to check each one's coverage area (think back to the original days of cell phone coverage). Most major US airports are now wired and a lot of other chains are putting in WIFI for their customers. McDonalds is even wirng their stores for access.
It's all very legal. Security isn't any worse than what you'd have with a typical home network connected to a broadband connection.
You might want to look at:
http://www.wififreespot.com/
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