Most of the cellcos have limits on their "unlimited" cellular internet
policies. Watch movies every night, stream multimedia, and you may get cut off.
If you use WiFi
signals that you can snag here and there, be aware that if they are not using WPA or better encryption, and you are not using a VPN service, everything you say may be viewed by others, except perhaps HTTPS pages. Including hackers who often set up those free access points for that exact reason. WEP is useless, WPA is somewhat better, WPA-2 and later versions still better again.
Real WISPs these days generally mean cellular internet
providers, although WiMax is supposed to deploy later this year form Sprint and some other carriers.