Last Friday I bought my first macbook pro; Lion was already installed. This of course means I had no previous keychain file to migrate and no initial entries in the existing keychain file. A few days after using mail, the keychain message would appear asking to enter password. If I also had Safari open, entering the keychain password and clicking OK resulted in Mail not responding no matter what was done (also the Keychain password box remained on screen). Attempting to force quit from the Apple menu was not possible as it was also froze. In both instances I had to shutdown the Mac by holding the power on/off down button down until it shut off the laptop (attempteing to restart did not work because it kept telling it could not restart because Mail needed to be exited).
After the restart completed, I reset my keychain file (no content of concern there), opened mail, entered the passwords, and so far (fingers crossed) it seems fine.
Apple: in my case nothing I did caused this problem. This is clearly a bug to me. Googling this problem shows users with keychain problems dating back many, many years. Certainly countless users have had serveral types of problems with the keychain. Please investigate and correct these problems. Thank-you