Straight away from their first phone, Jolla will be using Wayland as their display server rather than an X.Org Server, Mir, or some fbdev-based creation. Back in February the Finnish company released the Sailfish Alpha SDK but out today is a new updated SDK to provide Wayland support and updated Qt5 handling.
David Greaves of Jolla wrote on the Sailfish mailing list, "In this release we roll out a SailfishOS SDK that works like it's predecessor but supports Qt5 and QtQuick, with it's graphics pipeline switched from X11 to Wayland. Not only that but we've added update support so we can now roll out improvements - keep an eye on the Updates page in the Control Centre and watch out for QtCreator updates too."
Sailfish OS continues to be based upon the Mer Linux stack and will be using Wayland in a production capacity. Jolla developers spearheaded the project to make Android GPU drivers work with Wayland.