One of the podcasts I subscribe to is Gear Live to keep up-to-date with various technologies. The most recent episode talked about FreedomPop which is a service offering cheap pay-as-you-go and prepaid mobile data. They have a variety of hotspot options including cases for the iPod Touch and iPad which act as hotspots as well as a USB dongle and a typical MIFI type hotspot (same one I have for my Virgin Mobile hotspot). It uses Clearwire’s WiMax network for 4G and Sprint for 3G (note, not free, but still really cheap).
FreedomPop also claims to offer additional data for each person you refer (note, I do not have an affiliate/referral link).
If you live in a Clearwire WiMax coverage area this could be nice for some cheap or free 4G in airports. Otherwise YMMV.