Build 5600 of Longhorn is one of the first "release candidate"s of Longhorn. It's also one of the first builds to advertise itself as being "Vista".
The setup process has now basically been transformed into that which is present in the final version of Vista.
The splash screen is now identical to the one in Vista as well.
The login has been made to look like Vista's too. I believe the standard XP login is also present.
This build also contains the Welcome Center, as present in Vista.
Also included with this build is one of the first incarnations of the User Account Control annoyance.