Olaf,
I’ve tested idle timer support out today. WP8 definitely supports enabling and disabling multiple times within the same app session. I proved it by setting up the device to turn off the screen when idle for 30 seconds and having a timer switch between enabling and disabling the idle timer every 1 minute. Disabling the idle timer prevented the screen from turning off for 1 minute (as expected) and then re-enabling the idle timer made the screen turn off a short time later. Turning the screen on and re-disabling the idle timer prevented the screen from shutting off for 1 minute again. So, yeah, it definitely supports switching it back and forth.
If you want to do this for yourself in C#, then you can prevent the screen from turning off like this…
PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Disabled;
And you can switch on idle detection mode like this…
PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Enabled;
I’ll go ahead and support for it later this week anyways.