Precious in dual screen setups only, however. I found out by chance, that pressing Ctrl+< shifts the current window to the next screen. Isn't it wonderful?

Works in KDE 4.3, don't know about former versions.

Update: I'm sorry! The default setting is None for this shurtcut, which you can confuge in "Global Keyboard Shortcuts", section "Kwin", Key "Window to Next Screen". Obviously, i changed this long long long ago and forgot about it. Nevertheless, the shortcut is cool!