Perhaps I am not making myself clear.
1) I send myself an encrypted message. I must enter the keychain password to unlock my private key to encrypt the message. Perfect.
2) I check my mail and view the encrypted message. Before decrypting, I must again enter my keychain password. Perfect.
3) I select a non-encryped message. I can read it as expected, with no keychain prompt.
4) I re-select the encrypted message. I am not prompted for my keychain password; the message is still decrypted. Not perfect. For this method to be secure, I would expect to enter my keychain password every time I view a message.
5) I quit Mail.app. I re-start Mail.app. I select the encrypted message. I am not prompted for my keychain password; the message is still decrypted. Not perfect. I would expect that quitting Mail.app.
6) I quit Mail.app and lock the keychain. Now when I re-open Mail.app and select the encrypted message I prompted for my password.
Perhaps Mail.app caches decrypted messages until the keychain is locked.