In my system (Ubuntu Feisty (7.0)) it contains the following:

Ubuntu 7.04 \n \l

You can easily guess that \n is replaced by the hostname and \l by the number of the console.
Here is an overview over the functional characters:

\t	Current time in the format HH:MM:SS
\d	Current date in the format YYYY-MM-DD
\s	Name of the operating system
\m	Type of the processor
\r	Number of the kernel

The file /etc/motd (Message Of The Day) appears when a user logged in.

A picture for the door

The program Linux_Logo is able to create nice pictures e.g. for one of these files.
Under Ubuntu you can install Linux_Logo easily by typing sudo apt-get install linuxlogo into your bash. After that you can start Linux_Logo with the command linux_logo.
With some parameters you are able to affect the image. -a outputs everything in ASCII, -c paints a tux and -h outputs a complete overview over the possible parameters.
If you like the pictures you've created, you can create a new welcome file:

cp /etc/motd /etc/motd.copy
linux_logo -c > /etc/motd

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