@k8greene - I don't have a Kindle, but this is from a friend of mine who bought one in October:
"3G coverage is hot potato at the moment on the support forum. Apparently Amazon are tricking people into thinking the 3G coverage is worldwide, it’s not. It is apparently only where there is a 3G signal on the Vodafone network. Obviously most of the UK is covered and plenty of places abroad, just not everywhere."
Looking at this you can see that there's a significant chunk of the USA, and most of Canada, that you can't use it in:
http://client0.cellmaps.com/viewer.html?cov=2&view=intl