Can't unlock preferences - no password dialog
I've had this same issue for months. Before it was my old MBP since "upgrading" from Snow Leopard to Lion, and now again on my brand new MBP with Mountain Lion.  After some digging it looks like a fairly common problem, possibly to do with CAC/ID card readers.

Anyway, this worked for me (without a reboot!). From Terminal:

sudo pkill -HUP securityd


When I did this Mail apparently lost it's connection to the security framework and started prompting me for passwords; I just restarted it and it was good to go.