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.
The latest KDE 4 update 4.1.4 is compiling now in the proposed updates section of Kubuntu 8.10.
To install it enable Pre-released Updates in Adept, then under Changes click on Upgrade and Apply.
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.