PSP, Vita, and Vita TV
Due to the release of the PlayStation Vita TV device, Sony has updated their policy to allow for up to three portable devices on the same account. Previously, any given Sony account could only support up to two devices (a PSP and a Vita), but as of this month you'll be able to add a third option.
This is something I can only hope that Nintendo will explore in the future, with a true 3DS to Wii U account linking system -- not just the unification of currency. Should you need some help activating or deactivating a system, check out Sony's handy guide here.