Over a year ago we talked about Plymouth potentially replacing Ubuntu's USplash boot program and then it became an action item and there were plans for Plymouth integration in Ubuntu 9.10. Plymouth packages for Ubuntu also emerged back in March. However, in May it was then decided that there would be no Plymouth for Ubuntu 9.10 as instead the developers wanted to just focus upon making Ubuntu Linux boot so fast that such a graphical splash screen would not even be needed. Canonical called for a ten second boot time for Ubuntu, but in the 9.10 cycle they did go on to deliver a new sort of splash screen.
Ubuntu 9.10 didn't end up seeing integration of Plymouth as USplash and the new XSplash ended up being used. However, the Ubuntu development community and Canonical seemed to have changed their mind. Just uploaded to the Ubuntu Lucid repository for Ubuntu 10.04 LTS is in fact Plymouth.
Plymouth 0.8.0 ended up getting pulled into Lucid and its package is currently building. The package can be found on Launchpad. Don't expect to see a beautifully polished Ubuntu-ized splash screen just yet for Ubuntu 10.04, but you can begin rejoicing that USplash is finally being replaced by Plymouth.
To properly benefit from Plymouth you will need a graphics driver that provides kernel mode-setting support. Going back to Ubuntu 9.04 there has been Intel KMS support, but with Ubuntu 10.04 there is now proper ATI kernel mode-setting support when using the open-source driver stack. Those with NVIDIA hardware can use Plymouth and kernel mode-setting once the Nouveau support has landed in the kernel. Users of the proprietary ATI Catalyst or NVIDIA graphics drivers do not have access to kernel mode-setting support at this time and thus will not be able to benefit from a clean, flicker-free experience.