Packages for the May updates of the KDE Platform, Workspaces and Applications 4.8.3 are available for Kubuntu 12.04 in the official repositories. As long as the precise-updates pocket is enabled (this is the default) you should receive this release as part of your normal system updates.
Many thanks go to Jonathan Kolberg, Philip and Rohan for working on the packages!
Update: Now available in the standard updates repository. Adding a PPA is no longer necessary.