The Antimalware Engine team at Microsoft has posted a heads up to the Antimalware Engine Notifications blog that the engine used to scan for antimalware in Microsoft Security Essentials and Forefront Client Security will be updated in both products.

As part of regular update of our antimalware technology to address the latest in the threat landscape, MMPC is planning to release a new antimalware engine on 16 September 2010

Make sure you have these programs set to auto update so you get the new engine as soon as it is ready for download.