When will KDE SC 4.8.3 be available in the Kubuntu 12.04 official repositories?
It's already here, but you must enable the Kubuntu Backports PPA from Kubuntu to get it: Plasma and KDE Applications 4.8.3 Updates
Stable Release Updates
Kubuntu Updates PPA /1/ -> Ubuntu Proposed /2/ -> Ubuntu Updates /3/
Maybe soon ?
You can put this question to Kubuntu team.
To do this follow this LINK, with the Kubuntu contacts.
Or you can:
Follow this link.