Transparent screensaver for linux?

This question is old, but in case people stumble across it, I use xtrlock for this purpose:

$ apt-cache show xtrlock
[...]
Description-en: Minimal X display lock program
 xtrlock is a very minimal X display lock program, which uses nothing
 except the Xlib library.  It doesn't obscure the screen, it is
 completely idle while the display is locked and you don't type at it,
 and it doesn't do funny things to the X access control lists.

Changes the cursor to a padlock but otherwise makes no visible changes. To unlock, hit Enter, type password, hit Enter (there's no prompt).

This trick works for me:

$ xlock -mode blank -geometry 1x1

However, don't forget that for mission critical servers which hold private data, a screen saver is not enough. Screen savers are programs, they may eventually have bugs. Sometimes they segfault and that may be the reason for unwanted people to get access. During 10 years of linux I've seen xlock segfaulting a couple of times. Fortunately, only on my home workstation which was physically safe from intruders :-)