Just got back from a great week away with Lorraine and Bean on the Norfolk cost to see that KDE 4.1.2 is out for Lenny.
For those not in the know, Ana Beatriz Guerrero López one of the Debian KDE maintainers has been maintaining a site and repository of backports for KDE 4 for Lenny. The site is pretty basic but does detail how to install KDE 4 for Lenny and provides news of when the updates are out.
I would warn that the updates do take longer then say Kubuntu to be added (as in 4.1.1, 4.1.2 and as in they take a week or two not months) but then when they do appear they are rock solid and it is the work of a small group.
As far as I am concerned Debian KDE 4 rocks over Kubuntu.