Ubuntu Security :: How Secure Is Plain Old XScreenSaver W/PAM

I recently installed Xubuntu on a 5+ year-old Acer Aspire laptop. In spite of how the root, swap and home are volumes in an LVM2 group on top of a fully-encrypted harddisk, it's quite swift this computer isn't for me, however; it's for my old man. It's been my pet project for the past few weeks, as an exercise in making Linux both highly secure and user-friendly, so that even a technophobe like him could use it without fear of identity theft if it ever gets lost or stolen. I recently shipped it off to him...

The weakest part of security, however, I fear may be the screensaver, which is why I ask. I've searched and searched and cannot manage to find any information on security hardening practices (apart from banal advice such as "always choose a secure password" and "never log in to an X-session as root"). What I want to know is, just how safe is a locked screen? Also...is it technically possible with the right tools to get a core dump of a running computer and thus extract the block device encryption key from memory? I know that your run-of-the-mill thief will just try hitting the power button and throwing in a livedisk, but hey, you never know. Edit: found. [URL] I guess I need to tell him how to set the BIOS password to thwart that sort of thing next time I talk to him on the phone...