No, this is not an April Fool's joke! SEGA has indeed released the packshots for Sonic's Dreamcast swansong, Sonic Adventure 2. You can see both the North American and European box art below. It really captures the theme of good/hero, light/dark doesn't it?
If box art has now been publicised, it means that further news on the game's release can't be too far away. We'll bring you news of that as soon as we get it.
European Box Art:
USA Box Art: