If you run apt-cache search plymouth-theme, you’ll see which themes are available in the default repositories. This Youtube search has some videos of what is available. I chose the plymouth-theme-solar package, which has a nice solar flare animation. Next, run the following code, which I found here. Keep in mind that you need to change it to point to the plymouth theme you want to use.

sudo update-alternatives --install \
/lib/plymouth/themes/default.plymouth default.plymouth \
/lib/plymouth/themes/solar/solar.plymouth 100

(note: Line warping was only used to make it look pretty on a blog post.)

Next run sudo update-alternatives –config default.plymouth. You’ll see a numbered menu of the themes currently installed. Type the number for the one you want and hit enter. Finally, use sudo update-initramfs -u to apply your changes.

Here is the result:

There are not many themes available currently. However, once 10.04 is released and has been out for a little while, I think there will be many more options.