I recently signed up for Clearwire's CLEAR service. They have a MiFi component that does "4G" with fallback to 3G if necessary. This gives me up to about 3MBs, with portability (up to 3 hours on battery). There is no data limit for "4G", and you get 5GB per month on the 3G fallback network.
Anywhere I travel inside the US, I'm using my home network, and isolated from public networks.