With the release of KDE 4.0, it's time to start maintaining both trunk and branch versions of the code, so that bugfixes can be backported to the branch.
If you use the cs/cb/cmakekde scripts to manage your build environment, it's fairly easy to switch the context of these between branch and trunk.
I am using the following scripts (I have trunk in $HOME/kde/src and branch in $HOME/kde/src_4.0):
Seems to work so far.
Oh, and since all the cool kids are doing it: