When we were developing the CloudSamurai multi-tenant framework, we were aware that there is an official Microsoft project doing the same thing as CloudSamurai using the similar ARR approach. Of course, the MS official project must be more mature and more complete in features. Although, we didn't know when and how this official MS offering would be available.
In the latest Azure June 2012 release, the 'Azure Web Site' is unveiled. This is the MS-self hosted version of high-density share-IIS multi-tenant web site solution. It is expected that the customer-hosted version be released in the next big release. Here is the official description of 'Azure Web Site':
"Quickly and easily deploy sites to a highly scalable cloud environment with the frameworks and open source apps of your choice using Windows Azure Web Sites. Get started for free and scale as you go on a cloud platform that enables automatic scale-out options across shared and reserved instances for greater isolation and performance."
You should also check ScottG's blog for more information: