In Leopard, if you've changed your home path from your Mac's internal hard disk to an external hard disk, as in this hint, and maybe you turned off your drive or maybe it unmounted for no specific reason, you'll see that it isn't sufficient to turn it back on to see all your preferences back to normal.
Why? As an example, let's assume you had moved your home folder here: /Volumes/HD300GB/users/yourname. When Leopard realizes the drive is missing, it automatically creates a folder named /Volumes/HD300GB (in this example). Then when you logout/login or restart your Mac, Leopard will remount your original drive with a different name; in this example /Volumes/HD300GB 1.
Consequence: You'll not have access to your home directory and your desktop, preferences, etc. will be (falsely) lost.
Here's how to fix it:
Login as a different user (with admin rights) ...