The latest update for 14.04 changed the lock screen behaviour.
For a while during development of 14.04, the lock screen would not run the screensaver - just lock the screen.
The following links to someone who did not like this behaviour:
I'm one of the weridos who did like the new behaviour.
How do I change it back?
(P.S this was closed as off-topic a few days ago because 14.04 was not officially released. Now that it has been I'm assuming that it's OK to re-open this question).