This had bothered me as well, so I thought I would pass it along. The key step here is backup before you archive and install. You did do that, didn’t you?
macosxhints.com – 10.5: Keychain access after 10.5 archive/install
After archive/install upgrading to 10.5, Mail.app, Safari, etc. all complained about my keychain information missing. So I simply copied the file ~/Library » Keychains » login.keychain to ~/Library » Keychains » yourusername.keychain. After that, everything worked fine again.
[robg adds: There were some comments on the queue review site about this hint that I thought I should share. I haven’t seen this issue myself, but the comments covered a range from “Thanks for figuring out this simple solution. This had been nagging me since install” to “I think there should be some advice on backing up the original before doing this” and “Are we sure this is even the right thing to do?” So this tip may or may not help you, but at least one of our reviewers noted that it solved the same problem for him.]