With the release of KOffice 2.0, the Kubuntu team have provided packages for 9.04.
To install, simply open System Settings, Software Management, then Settings, and then Edit Software Sources. Go to the Updates tab and check the box marked 'Unsupported updates' (which is the standard Ubuntu Backports repository). After that, refresh and then install the package koffice-kde4.
***PLEASE NOTE***
As KOffice 2 is not yet feature-complete as compared to Koffice 1.6. More information is available on the KOffice announcement page.