Recently, I tried to automatically unlock an encrypted external USB hard drive on Arch Linux. I have done this by inserting an appropriate entry into /etc/crypttab. I chose the device to be unlocked by cryptsetup with a key file by UUID.
Udisks listens to udev events and sends dbus events, so you can make a "daemon" script, similar to that devmon shell that waits for your usb drive and then does what it needs to do.
-- дамјан
