Here's a review by Ed Bott, This guy knows Microsoft.
http://blogs.zdnet.com/Bott/?p=1067&tag=nl.e539From page two: "New signatures are published three times a day"
This sounds good.
And this answered my questions:
"Like most modern antivirus software, MSE relies on up-to-date signatures, but adds its own cloud-based twists. Contrary to some recent reports, this isn’t a cloud-based service. Instead, it offers a dynamic signature service that pushes signatures on a daily basis, but adds the ability to query the signature service when need to reduce the window of exposure to new malware. By monitoring for suspicious behavior, the service can query for a sample when necessary. Rootkit detection features target kernel-mode malware and can detect the sort of tampering in the kernel that is typical of rootkits."
The new name: Microsoft Security Essentials
Here's another review by Paul Thurrott, Someone who also knows Microsoft inside and out.
http://www.winsupersite.com/win7/mse_beta.asp"The Microsoft Security Essentials public beta will be made available in the US, Israel, and Brazil on June 23, 2009."