How can I show or hide boot messages when Ubuntu starts?

You would need to edit the file /etc/default/grub. In this file you'll find an entry called GRUB_CMDLINE_LINUX_DEFAULT. This entry must be edited to control the display of the splash screen.

The presence of the word splash in this entry enables the splash screen, with condensed text output. Adding quiet as well, results in just the splash screen; which is the default for the desktop edition since 10.04 (Lucid Lynx). In order to enable the "normal" text start up, you would remove both of these.

So, the default for the desktop, (i.e. splash screen only):

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" #Hide text and show splash

For the traditional, text display:

GRUB_CMDLINE_LINUX_DEFAULT=        #Show text but not the splash

For the splash, but the ability to show the boot messages by pressing Esc:

GRUB_CMDLINE_LINUX_DEFAULT="splash"

Or, finally, for just a (usually) black screen, try:

GRUB_CMDLINE_LINUX_DEFAULT=quiet   #Don't show Ubuntu bootup text
GRUB_CMDLINE_LINUX="console=tty12" #Don't show kernel text

After editing the file, you need to run update-grub.

sudo update-grub

For more details, see this: https://help.ubuntu.com/community/Grub2

I may be out of subject, but you can just press "escape" during boot, to show/hide the plymouth splash screen...

There's a simple way of hiding all boot messages. In /etc/default/grub, adjust the following:

GRUB_TIMEOUT=0 # Do not show the GRUB menu at all
GRUB_CMDLINE_LINUX_DEFAULT=quiet
GRUB_CMDLINE_LINUX="console=tty12" # Redirect the kernel output to another tty

After that, just sudo update-grub, and it should be done.