@Miroslav Glavic - A mailing list would be good but the advantage of a plugin is that each of your installations of WordPress emails you if it wasn't up-to-date.

A mailing list would only let you know that an update had been released, it would not remind you of how many blogs you have or what their current version number is. An email coming directly from each installation could also contain a link taking you directly to that installation's upgrade page.

Actually, I won't be surprised if email notifications are soon made a default feature, it strikes me as the easiest way to reduce the number of neglected blogs, taking the viral momentum away from these worms.