I had that issue. I had regular updates the first few weeks of using the new Mint 6 installation and then the lock went away. I would try manually and Mint update would open and tell me how many updates but the window is grayed out so I could select nothing. Had to force a close after 15 minutes.
I re-installed the OS.
Lindows, Linspire, Freespire, Ubuntu, Mint 15 Cinnamon, Mint 16 XFCE, Mint 17 Cinnamon 64 bit.