Today (August 14th) an update was released that, once applied, will block RSA certificates with keys less than 1024 bits. The software update was released to the Download Center.
The security advisory is located at http://technet.microsoft.com/security/advisory/2661254.
The KB article is available at http://support.microsoft.com/kb/2661254.
The update is available now to allow organizations to assess the impact of this update and to reissue certificates with larger key sizes, if necessary, before the update is sent out through Windows Update. Previous blogs may have mentioned it being released to Windows Update this month. That is no longer the case. The update is planned to be sent out through Windows Update on October 9, 2012.
Please refer to the KB article for direct links to download the update for your supported version of Windows.