you can stop someone from booting it up with a LILO password,
but that doesn't stop someone from putting the hard drive in
another machine and mounting it on the new host computer.
The only way that I know of to secure data against
physical theft is a crypto filesystem.
On my laptop, my important files go into a crypto fs.
I use cfs. It isn't an e-smith answer, but
"apt-get install cfs" did the trick for me on my laptop. Then
I looked in /usr/share/doc/cfs/README, and thee was a pointer
to
http://www.cryptography.orgrob