ORIGINAL: hg..
jon, the VIN will read something like this WPOZZZ99Z5S...............
The number after the fourth Z (in this example it is a 5) states the model year of the car. It could be a 4 since the 2005 model year came out in October 2004. Beleive me it's worth checking, I'm in the middle of a legal battle over this which has cost me thousands and will probaly cost me thousands more!
As mentioned elsewhere, if the number after the ZZZ, i.e. the 10th character of the VIN, is a 5 then it's a MY05 car regardless of when the car was registered. My GT3 is MY04 but was in fact registered in June 2003. This confused the Porsche Centre so much that they gave me the wrong service book for the car, i.e. a 2003 service book. This wouldn't have been such an issue if the servicing rules hadn't have changed between 2003 and 2004 such that my car is in fact on 2 year/12000 mile service intervals rather than 1 year/12000mile and I was invited and paid for an unnecessary annual check as a result (I never did get an apology from the Porsche Centre and I now choose to run two service books in parallel).