This document lists resources for the plymouth startup splash
General information on Plymouth
Info on scripting
The mailing list
Information on Plymouth from a design perspective
Part 1: introduction, creating testing environment
Part 2: theming basics
Part 3: scripting and animation
Part 4: advanced tips & tricks
Ubuntu-specific technical notes
You don't need to compile Plymouth from source to get its X11 interface. Just install plymouth-x11 package.
Ubuntu uses Debian alternatives system instead of plymouth-set-default-theme command. This means that you'll have to use sudo update-alternatives default.plymouth path-to-your-theme-here to switch themes. Themes are usually located in /lib/plymouth/themes/theme-name/theme-name.plymouth
There are many example themes in Ubuntu repository. Run apt-cache search plymouth-theme-* to get a list of available theme packages. You can also view them online (with screenshots).