In the screensaver dialog (windows 7) you can check the box that states "on resume, display lock screen." Is it possible to show the User Selection screen (tsdiscon.exe) instead?
Yes. This action can be done quite easily in windows 7. First you will need to enable your screensaver. Next you will need to access the local group policy and under computer configuration you would need to enable event logging for logon/logoff events. Finally, you would configure a task that would launch tsdiscon with a trigger from event id 4802 (screensaver invoked). I have this configured on my workstations and it works great.