[expert] Shadowing / intercepting X-sessions?

[Iwan van der Kleyn]

In a Citrix environment (Winframe or Metaframe) it is possible for an
administrator to "shadow" a user-session. That is: to monitor the screen of
a user and to take it over. Arguably a rather drastic tool (with regard to
privacy-matters) but quite convenient when helping "users-in-distress". Is
it possible to do the same with XFree86?

Thanks,
Iwan