As of today the KDE Frameworks 5 packages have been added to Arch Linux's "extra" repository. These KDE Frameworks 5 packages can coexist on the same Arch system as KDE 4. Arch Linux users can fetch these new packages from extra via kf5 and kf5-aids.
The KDE Arch contributor working on this, Andrea Scarpino, also hopes to have the current development packages of KDE Plasma Next packaged up in the next few days. Plasma Next initially isn't going into a main/extras repository but initially under kde-unstable and is prefixed from the rest of the system. Those wishing to learn more about these early "KDE 5" Arch packages can read Andrea's blog post.
Plasma Next is expected to ship its first official release in July while the KDE Frameworks should be stabilized in June.
Those not using Arch Linux but instead Ubuntu/Kubuntu can try out the latest next-gen KDE packages in an easy manner using Project Neon.