The KDE4Daily project was created to help people try out and test KDE4 on the run up to its 4.0 release without having to compile from source, or log out to boot from a LiveCD. Further, it aims to be updated nearly every day, so that people always have a mostly up-to-date (but, inevitably, occasionally broken) KDE4 install. Typical daily updates range from 20-50MB in size and take a few minutes to apply. KDE4Daily accomplishes these goals by using virtualization technology, with Qemu being the VM currently employed.
See here for details about the project: