HMC 4.0 has been Released To Web here and you can find the documentation here (UPDATE The documentation file works now).
Microsoft Solution for Hosted Messaging and Collaboration 4.0 offers hosting service providers with tools, tested best practices, scripts, and code samples designed to efficiently deploy messaging and collaboration services on multi tenant servers. The solution provides both automated and manual procedures for deploying, running and operating a hosted environment.
Most notably it now includes Exchange Server 2007 and WSS 3.0 hosting. If you are not familiar with the product you can read more about the solution here (Or better yet contact us at Inceptio A/S for an introduction / Architectural Design Session ;-)