I recently went through an episode where every time I turned around, my keychain would lock, and I would have to enter my password to several stacked-up "want to use my keychain" boxes. I ran Keychain First Aid (in the Keychain Access menu in Keychain Access), and nothing was wrong with the keychain. I also fiddled with all of the various options for when to lock the keychain, and nothing helped.
I searched here and everywhere, but couldn't find a clue as to the problem. But, after the umpteenth session of googling, I came across a brief mention that SSHKeyChain, a great little program that I use and love, might lock the keychain by default every time the screen saver activates.
I looked through the SSHKeyChain preferences, and eventually, I discovered that if you go to the Security tab and select "Use custom security settings," a bunch of additional options show up, including an "On screensaver" menu that allows you to specify "No action." This completely resolved the issue.
Admittedly, this is not a very exciting hint, but words cannot express the relief I felt when my screen saver quit and the little lock icon was still unlocked.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070927204456413