KDE 3.5.9 was released a couple of weeks ago and it is not yet in unstable. You might think the reason is because the KDE team is lazy, and actually we are, but the true reason is we could not upload it until we fixed some problems with Qt3. It will be uploaded from tomorrow, but in the meantime, if you can not wait like 24 hours, you can find packages of KDE 3.5.9 for both i386 and amd64 at:
deb http://people.debian.org/~ana/kde359/ ./
If you find problems, please send an email to the Debian KDE users list and remember restart KDE after upgrade kdelibs!
These packages are the same packages that will be uploaded to unstable, so you can upgrade safely. Anyway, do not expect big changes out of kdepim.
About KDE 4, 4.0.2 will be released this wednesday, so you will find the development platform in unstable and the rest of the modules in experimental from wednesday morning.
But if you read the latest Release Update, it included a nice sentence There may be a possibility to include KDE4 in lenny. Let me explain this :)
We, the KDE team, are planning start working in packaging the 4.1 branch and if it seems as good as we expect, we will move to package directly KDE 4.1 snapshots/betas/release candidate with the goal of at least shipping KDE 4.1 development platform in Lenny, and maybe whole KDE 4.1 replacing KDE 3.5.X. The details are still undecided, and it all depends of the evolution of KDE 4.1 and Debian release status.