passenger side, near the back of the sump (the flat plate that the oil plug goes into). it is beveled and probably so covered in crud that you can't see the number.
in the main image below, look at the top of the right side of the sump - you'll see beveled metal bit sticking out the side of the block - vin number on that:
http://www.pelicanparts.com/techarticles/Boxster_Tech/12-ENGINE-deep_sump/images_med/Pic1.jpg
*** beaten by the quick trigger finger of rp17!