Yesterday I upgraded my work Macbook Pro from OS X Tiger 10.4 to Leopad 10.5.6. It was not an entirely smooth experience. After upgrading to Leopard, it all appeared to work fine -- everything seemd intact. I ran the software updates tool, and it downloaded some updates. I needed to reboot.
I use FileVault to encrypt my home directory, to protect all the confidential data on it. When you reboot/shutdown the laptop under OS X 10.4 Tiger, it asks you whether you want it to free up space occupied by deleted files within the encrypted image. You can "Skip" or "Continue".
After installing the updates and rebooting, I elected to "Skip". When the laptop came back up, I could not log in to my account. I rebooted with the OS X 10.5 Leopard install DVD (hint: press Apple-C at start-up to boot off DVD), ran Disk Utility and tried to repair the encrypted .sparseimage in my home directory. No luck.
I ended up re-installing OS X 10.5 Leopard from scratch. Fortunately I had a backup on my Linux box (encrypted with encfs).
Couple of lessons learnt:
Set up an administrator account that does not use FileVault. All the help articles I've seen assume that you can actually log into your Mac under an admin account and run Disk Utility. I wasn't able to because my account was the only administrator account.
Don't use the "Skip" option with FileVault. NB: It appears they removed it in OS X 10.5 Leopard.