You can do this by installing and configuring Plymouth. Depending on your graphics card configuration, you might need some additional configuration as documented in the Debian wiki entry for Plymouth. But before you make all of those changes, try the following commands and see if they work for you.
Start by installing the Plymouth package.
apt-get install plymouth
/etc/default/grub and modify the
GRUB_CMDLINE_LINUX_DEFAULT parameter to have the following values.
Finally, update the grub config files.
Now when you boot you will no longer see the diagnostic messages. The messages are supposed to be logged to /var/log/boot.log but it's broken in squeeze. This page claims to have a patch: http://us.generation-nt.com/answer/bug-678982-plymouth-boot-log-not-created-help-207993181.html
Plymouth comes with several boot themes. The default theme is called "text" which will display a blue progress bar at the bottom of the screen during boot. If you'd like to change the boot theme, run the following command with your desired theme name. The
-R option is used to rebuild initrd which must be done when changing the theme.
plymouth-set-default-theme -R <theme>
To see a list of available themes, run the following command.