I posted this bug on 5/12/2013
The problem has now manifested on my Win7 PC as well. It coincided with the latest Windows auto update which got installed on my Win7 rig on or about 7/2/2013.
Some additional information:
The problem occurs 90 seconds after the screen saver goes off regardless of how long the screen saver is configured to wait before kicking in. So my screen saver will only ever run for 90 seconds and then it gets 'woken up'.
-- Axe22