Alternate title: 'for those who care about channels 12-13'. For background, see the earlier entry on wireless regulation.
If your wifi interface no longer supports channel 12 or 13, this may be because the kernel does not know the regulations for the domain code detected by the driver. Prior to Linux 2.6.34, regulations for 'US', 'JP' (Japan) and 'EU' (ETSI members) were built-in, but now there is only '00' (world, conservative regulations) which allows only channels 1-11 in the 2.4 GHz band. For any driver that detects specific European country codes, the 'EU' regulatory information was useless even in 2.6.32 (squeeze) and the unknown code would be mapped to the '00' (world) domain.
Since wireless regulations are subject to change from time to time, they are now provided by the wireless-regdb package and supposed to be loaded into the kernel by crda. These packages should have been included in squeeze, but for various reasons they missed it. But they are now available in squeeze-backports.