When you start up your mac and the login window pops up, you have a big shiny apple logo and the words Mac OS X. Below these words you usually see the name of your mac, whatever that may be. For most people that isn't very useful, as you have to have a lot of the same models knocking around to get confused between computers, and luckily since Mac OS X 10.4.3, this line has been made much more informative.

Since this version, you can click on the name of your mac to show different statistics about your machine. Each time you click, it will cycle through different bits of information that are particularly useful when troubleshooting problems.

The order of the statistics are as follows:

One click: Your OS X version number (e.g. Version 10.4.7)
Two clicks: Your OS X build number (e.g. Build 7M271)
Three clicks: Your mac's serial number (e.g. WN1511LHKNW)
Four clicks: Your mac's IP address (e.g. 196.254.0.1)
Five clicks: The status of any networked account
Six clicks: The date and time (e.g. Saturday, January 20 2007 4:02:31 AM GMT)
Seven clicks: Back to where you started, the name of your computer.

So you click a few times and set the information line to your favourite thing, but next time you log in, it has changed back to the name of your mac. To change the bit of information that appears first, you need to do a bit of tinkering in the Terminal (Applications/Utilities). Open Terminal and type the following command:

defaults write /Library/Preferences/com.apple.loginwindow AdminHostInfo info_name

Depending on which piece of information you want displayed, replace info_name with one of the following:

SystemVersion
SystemBuild
SerialNumber
IPAddress
DSStatus
Time

After doing this, hit return, and the next time you view your login window the information you chose will be displayed.