The 3G on a Kindle Keyboard is still going through Amazon, they just contract out the data towers through AT&T if I remember correctly.
On the devices that were sold before the ban, data was capped at 50 MB per month. I for one have always assumed Amazon did not want to completely remove a feature that the device was marketed as having.
On newer devices, only Amazon's servers can be accessed. (Both wikipedia and Bing Translate are and probably always have been proxied through Amazon's servers.)
I imagine this restriction is probably managed through your serial number, which is how Amazon usually identifies a specific device.
You could try switching out the SIM card, but I doubt it would work. And faking the serial number is a non-starter. (It
has been done a few times, between two of the same model device. Handy if your 3G Kindle breaks and you want to move the 3G card into a WiFi model. It's also at the far end of really-complicated-debricking-tactics.)