KDE 4.2 Release Candidate 1 has been released and packages are available for Kubuntu 9.04 "Jaunty". Packages for Kubuntu 8.10 "Intrepid" are compiling and will be available shortly, check back soon if they are not all available when you read this. Read more for details.
KDE 4.2 has been released. Jaunty (current development release) users can do a full upgrade to get the latest packages. Packages for 8.10 (Intrepid) are in intrepid-backports.