It happens only in itunes, I usually ignored it but sometimes it gets irritating. I tried repairing permissions and it still happens. Guess I have to install itunes again.
Here's an older thread with some solutions...none appear to fix the problem completely, unfortunately. But there are some troubleshooting suggestions that may clear it up and some easier workarounds than restarting if it doesn't.
That has been happening to me since iTunes 7 was released. Its not critical, but the way to get it to clear up is to just restart iTunes. I don't think they are too concerned right now about getting it fixed.