I have written a tutorial about how to change Plymouth theme,the initial splash screen in Ubuntu 10.04.But there are only a few themes in Ubuntu default repository.Here I found Mac OS X splash screen theme.After install the theme,Ubuntu boot-up and shutdown splash screen look like this:

Install the Mac OS X Theme:

First,download the theme from this page

It is a .zip file,right-click on the package and unzip it.Then copy and paste the Paw-OSX folder into /lib/plymouth/themes/ directory.You need super user to do this copy and paste thing,so use sudo nautilus command to open up file browser to get super privilege.Or,use following command to copy and paste the theme to /lib/plymouth/themes(I downloaded and saved the zip file in Downloads/ folder):

cd Downloads
sudo cp -R Paw-OSX/ /lib/plymouth/themes

Now,install the Mac OSX theme with following commands.

sudo update-alternatives --install /lib/plymouth/themes/default.plymouth default.plymouth /lib/plymouth/themes/Paw-OSX/paw-osx.plymouth 100
sudo update-alternatives --config default.plymouth

It outputs all current available themes.You just need to press the number at fond of Paw-OSX theme and hit Enter to use it.

Finally,run this command to apply the change.

sudo update-initramfs -u

Done,reboot and see the new splash screen!