At my last work place, the Sys Admins implemented a new policy such that any workstation left unattended for 5 minutes would automatically lock, and then show the screensaver.
That REALLY annoyed some people! For example, you could be reading a document on-screen, and if you didn't move the mouse or hit a key for 5 minutes, your workstation would lock.
It was a real pain for the reception staff, who generally had to unlock their workstation whenever a customer walked in.
However, the reasons behind the move were sound, and people fairly quickly adapted to the change.
As for shared accounts, generally a big no-no.
My view is that the sooner we start using Smart Cards (or some other 2-factor authentication) the better. Take your card with you when you leave your workstation, and it automatically locks. I like the idea of using bluetooth on the mobile phone, that is clever.
I will admit to still running my workstation as a member of the local Administrators group though.