Some people are reporting an issue with iTunes 10 in which the screensaver starts while they are using iTunes' visualizer in full-screen mode on connected devices like HDTVs. Prior to iTunes 10, the visualizer would take precedence on the screen display, not allowing the screensaver to start when playing, similar to a full-screen video. Apple Support Discussions user "megadot" writes:
I have myMini connected to my flat screen TV. I have my screen saver set to come on after the minimum time to protect my plasma panel. Since upgrading to iTunes 10 my screen saver starts after 3 minutes when I'm using a full screen visualizer. I use the visualizer whenever I'm listening to music. This didn't used to happen and it's really annoying. The same thing happens with iTunes 10 on my desktop Mac and my MacBook Pro. Is there a way to stop this happening without turning the screen saver off?
Several other commenters have chimed in, citing similar symptoms. Testing the issue on my MacBook Pro, with no displays connected, I found it happened as described. I set the screensaver wait time to 3 minutes and put the iTunes visualizer in full-screen mode. At the 3-minute mark, the visualizer blacked out and the screensaver took over.
Waking my MacBook Pro deactivated the screensaver and put the visualizer in a window. The result was the same when activating the screensaver with a preset Hot Corner.
After testing every combination of screensaver and visualizer settings I could think of, I concluded that this appears to be not a bug, but rather a change to iTunes 10. Most likely, Apple engineers found that allowing the visualizer to continue to play at full screen was depleting the battery life of portables too quickly. In order to help protect battery life, any screensaver settings trump the visualizer.
Because this is an inherent function of iTunes and Mac OS X, there is no "fix" other than turning off your screensaver. A work-around, one that I use, is turning off your automatic screensaver setting and using a Hot Corner to activate your screensaver manually. Navigate to System Preferences > Exposé & Spaces > Exposé. Choose the corner you wish to have as a Hot Corner and choose "Start Screen Saver" in the drop-down menu of that corner.
As many of the users in the ASD forum have already done, you can leave specific feedback for Apple about this issue by clicking iTunes in the iTunes menu bar and selecting Provide iTunes Feedback.