I'm using Ubuntu 10.04 (amd64) with the latest KDE version and i noticed, that each time i log in my monitor goes black for a few seconds. After the 2 or 3 seconds the normal screen reappears.
I have this issue on two different machines: one with 2 monitors and one with only one monitor. Both machines are using the nvidia driver.

Any ideas?

This doesn't sound like a KDE issue, I think this is an issue with mode setting, where the graphics driver switches modes it goes black briefly. Most drivers support what is called kernel mode setting, which avoids this issue. Nvidia does not and has so far refused to even consider implementing it.