On my Panny the problem is Sky. If i'm watching a recording it reverts to the Home page and the preview box prevents the screensaver kicking in. So I have to be mindful not to wonder off when a recording is playing.The screensaver on my LG kicks in after half a minute when I use some apps, youtube, netflix but not iPlayer or if I pause a BD.
If its me or the wife watching, we just revert to live TV if we walk away. But if I've left something on for the kids I can't do that. Usually its ok, but sometimes when they're watching a series, if an episode is missing it will go to the home screen instead of playing the next available one. Fortunately they usually shout and my eldest (5y/o) already knows to press the backup button to exit the Home screen.
But its a worry none the less.