Microsoft Malware Protection Center (MMPC) is planning to release a new antimalware engine on 28 Nov 2011. Engine Version will be in the range of 1.1.790x.0.
Affected products: Microsoft Security Essentials, Forefront Client Security, Forefront Endpoint Protection, Windows Intune Endpoint Protection.

So I think, the new Beta of Microsoft Security Essentials will be made available only after this is released. We had already posted about -

(Source: New antimalware engine planned for release today ! )