people have also hacked the kindle, the savory install that was mentioned is one of the available hacks. there is also one to tinker with the screensaver and one (for the kindle DX) to modify the browser
these install the hack binaries on the kindle, so the mechanism is known. one of the people working on this has said that he has been able to get a root shell on the device.
unfortunantly the people working on this are afraid that if they make their work too public, people will try to hack the kindle to use it as a computer internet connection, and in the process cost amazon enough money that amazon will cancel the free cell-based Internet access that the devices currently enjoy.
I suspect that this is not really a problem for a couple of reasons.
1. Internet access goes through a proxy at amazon, which limits (to some extent) what you can get to. However, I don't know if this is mandatory routing based on IP or just configured in the kindle browser
2. I suspect that the contract with sprint for the 'wispernet' service only allows the kindles to use otherwise idle bandwidth (in other words, it really doesn't cost sprint anything)