Here are few articles that can help you deciding about the security architecture of SharePoint Products and Technologies.
Insight into Security Model using Principal and Identity Objects in .NET
http://www.codeproject.com/dotnet/SecurityModelDotNet.asp?df=100&forumid=24415&exp=0&select=721205
Security Architecture for SharePoint Products and Technologies – Resource Kit Sample Chapter on Security in SharePoint
http://www.microsoft.com/technet/prodtechnol/sppt/reskit/c0661881x.mspx
Using SharePoint Web Services to Change Subsites Permissions
