I believe you are right in that it is the in-service date, not the production date.
My was put into service in June 2006 and has 11,000 miles on it.
My 8yr/80,000 goes until June 2014 or 80,000miles which ever comes first.

So purchase date of the warranty doesn't matter, only the in-service date.
Most car manufacturers do extended warranties this way. It's the 3rd party warranties that tend to go by the warranty purchase date.
-Steve