Apple made an announcement to the effect on its website. The announcement reads “Apple has determined that, in very rare cases, the battery in the iPod Nano (1st generation) may overheat and pose a safety risk. Affected iPod nanos were sold between September 2005 and December 2006.”
The batteries supplied by a particular supplier alone have been noted to be the cause of the trouble and only those units with batteries – identified by the appropriate serial numbers – from the supplier are going to be replaced. The replacement is expected to take some six weeks or so from the date the unit is received by Apple.