Daring Fireball has some thoughts Regarding the 80 GB iPhone Capacity in the Activation and Sync Video.

(a) The demo screencast was recorded using a super-secret pre-production iPhone 2.0 with 80 GB hard drive; or (b) the original 4 and 8 GB iPhones are still so secret that the screencast was recorded using an 80 GB iPod hacked to resemble an iPhone in iTunes.

See this post on the unofficial apple weblog about the mysterious 80G space.