Installing more than one desktop environment will sometimes change the
splash screen to another image or even have no splash screen show, depending
on what was added. Mine will sometimes show, Ubuntu, Kubuntu or Xubuntu. It
all depends on the order in which I installed KDE, GNOME or XFCE desktops. I
have also had no splash image from time to time. It never unsettles me as I
am used to changes. I could change the splash image but never care enough to
bother.

It could also be due to a change in graphics drivers. Ubuntu 9.10 and 10.04
use KMS and 10.04 changed the boot loading process using Plymouth to speed
things up. Depending on your graphics card a different driver may be used
early on and then switch when X is loaded. My Nvidia card does that. It uses
a vanilla graphics driver and then loads in the proprietary driver. Things
look hideous when the splash screen does show, but everything looks okay
once I get to the login screen.

As long as the OS works don't worry is my advice.