In older versions of Mac OS X, simply clicking on your computer name in the login window would cycle through displaying lots of useful system information. Unfortunately this feature has been removed in Lion, but you can bring it back to a certain extent. Open up Terminal (located in Applications/Utilities), paste in the following line and press Return:
sudo defaults write /Library/Preferences/com.apple.loginwindow AdminHostInfoHostName
Now when on the login screen, clicking on the time in the top right will cause the computer name to appear, followed by the OS version and build number, followed by the IP address.