I ordered the international 3G+WiFi version from the US, according to the debug information the home network for the simcard is in the UK (possibly Vodafone), and it uses an irish proxy to reach the web (amazon datacenter in dublin).
The network it will use is different in each country. The full list of enabled Mobile Network Codes are the following: 20810 24008 26801 20404 27801 22210 27201 20205 26202 21401 23801 22801 20601 27001 27402 23201 24405 310410 60202 28602 22601 23003 21670 26001 50503 45406 40420 40411 46000 65501 33805 25099 24201 27602 29340 23101 28401 25503 24802 24602 42501 60400 41601 41902 52018 52503 51011 50219 63902 72231
You can lookup the names here:
http://en.wikipedia.org/wiki/Mobile_Network_Code
Forbidden local networks: 23410 23433 23430 23420
Anyway I can browse the whole web from the foreign network 21670.