The Personal Package Archive for Plymouth (until it has made its way into Ubuntu) can be found at Launchpad.net. There is Plymouth, libplymouth, and the libplymouth development files available. The Plymouth packages are based upon the Plymouth Git code as of February 7th, which means it doesn't include some of the recent Plymouth work. Unless you build a kernel with kernel mode-setting support (the Ubuntu kernel PPA doesn't yet have KMS enabled), you will need to set a VESA mode option for your system (using the vga= option) from GRUB manually, which also means you will not see a flicker-free experience. The Plymouth PPA packages don't yet have a unique plug-in for Ubuntu, but by the time Ubuntu 9.10 comes around the Ubuntu artists will hopefully have a distinct splash screen.
Intel kernel mode-setting will be enabled on Ubuntu 9.10 as it was pushed into the mainline kernel (beginning with Linux 2.6.29), but the Nouveau KMS or ATI kernel mode-setting haven't yet entered the kernel and it may not in time for the Karmic Koala.