I updated my old Powerbook [previously running Tiger] to Leopard yesterday. Simple upgrade: backed the entire drive up to the backup partition, dropped Leopard in, ran the Upgrade Mac OS X install, and walked away for an hour and a half while it chugged. It booted up fine, I installed the available software upgrades — Apple, do we really have to download all of iTunes and Quicktime every damned time? — and rebooted again.
Launched Safari, and was presented with a dialog that told me that there was no Keychain file. I told Keychain Access to verify my Keychain, and it told me that the file was missing the extension. Turns out that Safari was right, and there was no keychain file at all. I rooted around in the backup, copied the old Keychain over, and re-ran Keychain first aid; it repaired the keychain, and all seems well now.
Edit on Nov 12: I guess all isn’t fine; the laptop requires the keychain to be unlocked the first time Safari is launched, and creating a new keychain and deleting the old one didn’t seem to fix it. I’ve seen various solutions for this posted on the Apple forums and around the net, and I’ve yet to find one that fixes my exact problem, but I haven’t spent more than 15 minutes or so looking into it.