Looks like Microsoft released Direct Access Design Guide on their download site.
As one of the most valorous features on Windows Server 2008 R2 it should be a really turn-point on the subject of Remote Access over the next few years..
This guide provides information about advantages, disadvantages, requirements, recommendations, and design considerations for deploying DirectAccess. It answers the “What,” “Why,” and “When” questions a deployment design team might ask before deploying DirectAccess in a production environment.
You can obtain the document here http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=647222d1-a41e-4cdb-ba34-f057fbc7198f