As well as the Dreamcast, it's also the 15th anniversary of the original PlayStation's US release. The Japanese anniversary site is still active, as there doesn't seem to be anything from SCEA.
There will be something interesting things on the PlayStation Store, according to the us official blog http://blog.us.playstation.com/2010/09/09/playstation-store-celebrates-15-years-of-innovation/
In Japan, campaigned nothing except an Anniversary site in last year. photo of PS Catalogue (Sep, 1994) and Demo Ridge Racer Grand Prix (PAPX-90001) oldest I own ;