My organisation has recently deployed Windows 7 Professional. Users who log onto these machines have a blank screensaver and a lockout idle time of 10 minutes, deployed via group policy. After the upgrade to Win7 Pro, I have ONE user who's computer will not go to screensaver and lockout after 10 minutes. I began troubleshooting whether this was a result of conflicting policies, but even after making a new user, everything seems to check out. We are deploying through our domain, the gpresults show both the default and the enhanced screen saver policies. When I look at the screensaver settings in the properties windows it shows the correct blank screensaver and the wait is set at 10 minutes and the logon screen at resume is checked. They are also all grayed out so that they cannot be changed. Everything seems normal other than it will not start after 10 minutes of sitting idle.
Check to see if the issue is specific to this user on their computer by logging into the computer as another user. If the issue persists than your issue is caused by something on the machine, if it does not persist than your issue is related to the user. If user related, try deleting their profile and letting it get recreated during the subsequent login.