Jan Dubois posted the results of his dive into Win32 localtime behaviour on p5p:

What I learned about Windows and Daylight Saving Time today

Basically, if $ENV{TZ} is set, MSVCRT.dll does the calculations itself and the latest version of MSVCRT as of today hasn't been updated to account for the new USA DST rules. There seems to be a hotfix for that.

If $ENV{TZ} is not set, the current rules will apply but also be applied to the past.

If $ENV{TZ} is not set, and you're running Vista/LongHorn, the current rules will apply and not be applied to the past, as Vista and upwards support a database of historical changes.