Sometimes you might install (say) kubuntu-artwork-usplash, when really you want the ubuntu usplash screen (provided by usplash-theme-ubuntu). To restore the original,

sudo apt-get remove kubuntu-artwork-usplash

This has actually happened to me a few times. Before, I think I had to muck about with update-initramfs, update-alternatives, ... But now it seems much simpler. I suppose if you really want to keep the kubuntu one then

sudo apt-get install --reinstall usplash-theme-ubuntu

might work, though I haven't tried that.

Edit: Apparently it's kubuntu-artwork-usplash
Also, this was done on 8.10, might not work on later versions.