An update was released for Dapper on 21 August 2006 UTC, which has been found to cause problems on certain systems.
A subsequent update published 17 hours later corrects this, so if your system is fully up to date now and you have no obvious graphical system failures, then you are highly unlikely to be affected. However, delays to the system of update distribution mean that you should make sure you have fully updated before rebooting your computer. You can read more about the issue to ensure you will not be affected, and learn what steps we are taking to ensure this does not happen again.
http://www.ubuntu.com/FixForUpgradeIssue