KDE 4.1 was released as a RC recently and will soon be released. While it will be a very usable and stable desktop environment ready to be used almost everywhere most users with NVIDIA cards will not be pleased: their proprietary driver spoil the fun.
You often hear that proprietary kernel modules are a problem only for RMS zealots. (I do believe that RMS has a lot of good points and I’m rather prefering the term Free Software over Open Source). But proprietary modules are not only a theoretical problem, but also cause practical problems.
Like not being able to fix a problem or implement a feature in such a module. As this is the case now with nvidia proprietary kernel module causing problems with the current KDE 4.1.