Um, yes it does! Win8 Defender now includes Security Essentials.
Here is the official word direct from Microsoft “Windows 8 Release Preview comes with Windows Defender, which has been improved in Windows 8 to provide the same level of malware protection as Microsoft Security Essentials. You do not need to install Microsoft Security Essentials and it will not work with Windows 8. Microsoft will continue to provide and improve Microsoft Security Essentials for Windows 7, Windows Vista, and Windows XP. If you’re upgrading to Windows 8 Release Preview, you should uninstall Microsoft Security Essentials first. For information on uninstalling Microsoft Security Essentials, see How to manually uninstall Microsoft Security Essentials.
Microsoft is actively working with antimalware app developers to ensure the availability of high-quality antimalware apps on Windows 8. If you prefer to use something other than Windows Defender, visit the Antimalware apps for Windows 8 Release Preview page to find out what works with Windows 8 Release Preview.”