It only affected 2.5L engines, and they do make them starting in the later end of the year before, but not all the cars. Unlike the other car companies that release a 2010 model in June of 2009. That is why a Boxster sold as a '99 might have a sticker on the door with a 10/98 build date.
More people custom order their Porsche versus just buying one off the lot, so building them all in advance wouldn't make much sense. For instance, my Boxster was custom ordered by the previous owner in 1998, it arrived in March with a January build date.
