I think it was mentioned a thread earlier in the year. At the auto show in NY they referenced the launch of the 718 as being a '20th anniversary' of the Boxster. Which is funny...."Happy Birthday, here's a turbo, but we're taking away two of your cylinders!"
The first models were sold in Germany in October of 96. But not sure on customer delivery (according the book 'Porsche Boxster' by Brian Long).
As far as the US was concerned, the first press event was in AZ in 1996 as Markus said above. The first public launch was the LA auto show in January of 97, with cars appearing in US dealers a few weeks after that.
Road and Track did a good article that I think was already circulated here
A Look Back at the Car that Saved Porsche: 20 Years of the Boxster
And for nostalgia's sake, here is Motor Trends first review...
Porsche Boxster - Road Test - Motor Trend Magazine
My guess is that Porsche will do some anniversary models for it's 25th birthday, with a 6-cyl Special Edition among others.