Interesting subject. There are wacky nerds (God love 'em) who will try to hack anything. And (in the endless cat and mouse game) I would guess that there will probably be Kindle firmware updates (done automatically through the wireless network) that will negate some of these hacks, or the functionality of hacks employed.
The Kindle 2 uses Whispernet so this map indicates theoretical coverage area. One of the links above observes that hackers should "Bear in mind also that Amazon know where you live. They know your Kindle's serial number and thanks to the built in GPS, they know where you are right now."
That isn't exactly accurate. Kindle, to my knowledge, doesn't actually have GPS, but it uses the cell tower triangulation scheme similar to that used in Google Latitude.
Since many of the posts above were made in March of '09 I expect this to be an area where the knowledge is still growing. I would be very surprised if these hacks will ever evolve to the point where you could use a Kindle to connect free to a EVDO internet connection for free web surfing in your car, but I love the thought process that went into imagining the possibility!