There is a security exploit going around that affects installations below the current latest version of WordPress 2.8.4. If you notice any weird code in your permalinks, and haven't updated to the latest WordPress version, then you've probably already been hit. Upgrading to the latest version of WordPress (2.8.4) should protect you from this latest exploit. More info here: http://wordpress.org/support/topic/307660
Beside strange permalinks, there is another sign : extra hidden admin account. More Info at : 1. http://zenverse.net/old-wordpress-version-under-attacked-upgrade-now/ 2. http://lorelle.wordpress.com/2009/09/04/old-wordpress-versions-under-attack/ Upgrade now !!
Yes, I forgot to mention the extra admin account which is very scary. If you notice your user count doesn't add up, and you have 1 less user showing up in your user list than the count, then you probably have a hidden admin account which needs to be deleted from the database manually ASAP. I also probably should have said something like "Security Exploit" in the title. If you've been lazy about your upgrades, now is a good time to stay up to date.
Smart move. I know there are a lot people who have numerous WP blogs and can't manage to update them all with every new release (sometimes even me) so hopefully this will serve as a wakeup call to anyone not up-to-date.
It is a real pain to keep up with, but at this point it should be manadotory for all of us to do almost immediately. If you are not already signed up, go to wordpress.org/download/ and enter your email address into the Release Notification mailing list (right side of page). This way you will get email reminders about the updates. Good luck
I don't think they use that newsletter very often. I remember getting one email about a new release a long time ago, but that's it. You could try subscribing to the WordPress.org development blog or follow @wordpress on Twitter for the latest info on new releases. Like ads2help said, there is usually a notice displayed in your admin panel soon after a new version is released (and yours is outdated).