I have now bought a Macbook Air with snow lepoard and have exactly the same problem. The mac is picking up country code TW, this then activates software in the mac to limit it to the channels allowable for use in Taiwan i.e. 1 to 11. You can see this in the "about this mac, advanced and AIrport section". This is happening with my 2820 and an AP 700. I also have a home plug with wireless N and the Mac also seees the country code as TW when connected to it. I have a suspicion that it may be getting that from the router not the home plug as I could have sworn that it was showing another country code when the link back to the router was down (cable loose at router). Tried disconnecting the cable again but could not reproduduce the scenario. I havent tried power cycling the home plugs with no connection to the router yet. Could be that the AP 700 is also getting TW from the router but have not tried experimenting with that yet.
The Telnet command "sys cc" displays the router wireless region in my case this reports:
"country code: 0x 9 (uk)"
"Wireless region code: 0x30"
So it would appear this is not being picked up and broadcast by the router.
Anybody got any further with this problem?